Call the cancellationTask of Scheduled directly

### Motivation:

In my previous PR apple#2010, I was able to decrease the allocations for both `scheduleTask` and `execute` by 1 already. Gladly, there are no more allocations left to remove from `execute` now; however, `scheduleTask` still provides a couple of allocations that we can try to get rid of.

### Modifications:

This PR removes two allocations inside `Scheduled` where we were using the passed in `EventLoopPromise` to call the `cancellationTask` once the `EventLoopFuture` of the promise fails. This requires two allocations inside `whenFailure` and inside `_whenComplete`. However, since we are passing the `cancellationTask` to `Scheduled` anyhow and `Scheduled` is also the one that is failing the promise from the `cancel()` method. We can just go ahead and store the `cancellationTask` inside `Scheduled` and call it from the `cancel()` method directly instead of going through the future.

Importantly, here is that the `cancellationTask` is not allowed to retain the `ScheduledTask.task` otherwise we would change the semantics and retain the `ScheduledTask.task` longer than necessary. My previous PR apple#2010, already implemented the work to get rid of the retain from the `cancellationTask` closure. So we are good to go ahead and store the `cancellationTask` inside `Scheduled` now

### Result:

`scheduleTask` requires two fewer allocations

Sources/NIOCore/EventLoop.swift

@@ -21,18 +21,13 @@ import Dispatch
 /// will be notified once the execution is complete.
 public struct Scheduled<T> {
     /* private but usableFromInline */ @usableFromInline let _promise: EventLoopPromise<T>
+
+    @usableFromInline let cancellationTask: (() -> Void)
 
     @inlinable
     public init(promise: EventLoopPromise<T>, cancellationTask: @escaping () -> Void) {
         self._promise = promise
-        promise.futureResult.whenFailure { error in
-            guard let err = error as? EventLoopError else {
-                return
-            }
-            if err == .cancelled {
-                cancellationTask()
-            }
-        }
+        self.cancellationTask = cancellationTask
     }
 
     /// Try to cancel the execution of the scheduled task.
@@ -42,6 +37,7 @@ public struct Scheduled<T> {
     @inlinable
     public func cancel() {
         self._promise.fail(EventLoopError.cancelled)
+        self.cancellationTask()
     }
 
     /// Returns the `EventLoopFuture` which will be notified once the execution of the scheduled task completes.

docker/docker-compose.1604.53.yaml

@@ -18,22 +18,22 @@ services:
   test:
     image: swift-nio:16.04-5.3
     environment:
-      - MAX_ALLOCS_ALLOWED_1000_addHandlers=46050
-      - MAX_ALLOCS_ALLOWED_1000_addHandlers_sync=39050
+      - MAX_ALLOCS_ALLOWED_1000_addHandlers=48050
+      - MAX_ALLOCS_ALLOWED_1000_addHandlers_sync=41050
       - MAX_ALLOCS_ALLOWED_1000_addRemoveHandlers_handlercontext=9050
       - MAX_ALLOCS_ALLOWED_1000_addRemoveHandlers_handlername=9050
       - MAX_ALLOCS_ALLOWED_1000_addRemoveHandlers_handlertype=9050
       - MAX_ALLOCS_ALLOWED_1000_autoReadGetAndSet=25050
       - MAX_ALLOCS_ALLOWED_1000_autoReadGetAndSet_sync=0
       - MAX_ALLOCS_ALLOWED_1000_getHandlers=9050
-      - MAX_ALLOCS_ALLOWED_1000_getHandlers_sync=36
+      - MAX_ALLOCS_ALLOWED_1000_getHandlers_sync=38
       - MAX_ALLOCS_ALLOWED_1000_reqs_1_conn=30400
       - MAX_ALLOCS_ALLOWED_1000_tcpbootstraps=4050
       - MAX_ALLOCS_ALLOWED_1000_tcpconnections=163050
-      - MAX_ALLOCS_ALLOWED_1000_udp_reqs=12050
+      - MAX_ALLOCS_ALLOWED_1000_udp_reqs=12000
       - MAX_ALLOCS_ALLOWED_1000_udpbootstraps=2050
-      - MAX_ALLOCS_ALLOWED_1000_udpconnections=89050
-      - MAX_ALLOCS_ALLOWED_1_reqs_1000_conn=428050
+      - MAX_ALLOCS_ALLOWED_1000_udpconnections=88850
+      - MAX_ALLOCS_ALLOWED_1_reqs_1000_conn=428000
       - MAX_ALLOCS_ALLOWED_bytebuffer_lots_of_rw=2050
       - MAX_ALLOCS_ALLOWED_creating_10000_headers=0
       - MAX_ALLOCS_ALLOWED_decode_1000_ws_frames=2050

docker/docker-compose.1804.52.yaml

@@ -18,21 +18,21 @@ services:
   test:
     image: swift-nio:18.04-5.2
     environment:
-      - MAX_ALLOCS_ALLOWED_1000_addHandlers=46050
-      - MAX_ALLOCS_ALLOWED_1000_addHandlers_sync=39050
+      - MAX_ALLOCS_ALLOWED_1000_addHandlers=48050
+      - MAX_ALLOCS_ALLOWED_1000_addHandlers_sync=41050
       - MAX_ALLOCS_ALLOWED_1000_addRemoveHandlers_handlercontext=9050
       - MAX_ALLOCS_ALLOWED_1000_addRemoveHandlers_handlername=9050
       - MAX_ALLOCS_ALLOWED_1000_addRemoveHandlers_handlertype=9050
       - MAX_ALLOCS_ALLOWED_1000_autoReadGetAndSet=28050
       - MAX_ALLOCS_ALLOWED_1000_autoReadGetAndSet_sync=0
       - MAX_ALLOCS_ALLOWED_1000_getHandlers=9050
-      - MAX_ALLOCS_ALLOWED_1000_getHandlers_sync=36
+      - MAX_ALLOCS_ALLOWED_1000_getHandlers_sync=38
       - MAX_ALLOCS_ALLOWED_1000_reqs_1_conn=30450
       - MAX_ALLOCS_ALLOWED_1000_tcpbootstraps=4050
       - MAX_ALLOCS_ALLOWED_1000_tcpconnections=164050
       - MAX_ALLOCS_ALLOWED_1000_udp_reqs=12000
       - MAX_ALLOCS_ALLOWED_1000_udpbootstraps=2050
-      - MAX_ALLOCS_ALLOWED_1000_udpconnections=89750
+      - MAX_ALLOCS_ALLOWED_1000_udpconnections=89800
       - MAX_ALLOCS_ALLOWED_1_reqs_1000_conn=433000
       - MAX_ALLOCS_ALLOWED_bytebuffer_lots_of_rw=2050
       - MAX_ALLOCS_ALLOWED_creating_10000_headers=0

docker/docker-compose.2004.54.yaml

@@ -18,19 +18,19 @@ services:
   test:
     image: swift-nio:20.04-5.4
     environment:
-      - MAX_ALLOCS_ALLOWED_1000_addHandlers=46050
-      - MAX_ALLOCS_ALLOWED_1000_addHandlers_sync=39050
+      - MAX_ALLOCS_ALLOWED_1000_addHandlers=48050
+      - MAX_ALLOCS_ALLOWED_1000_addHandlers_sync=41050
       - MAX_ALLOCS_ALLOWED_1000_addRemoveHandlers_handlercontext=9050
       - MAX_ALLOCS_ALLOWED_1000_addRemoveHandlers_handlername=9050
       - MAX_ALLOCS_ALLOWED_1000_addRemoveHandlers_handlertype=9050
       - MAX_ALLOCS_ALLOWED_1000_autoReadGetAndSet=25050
       - MAX_ALLOCS_ALLOWED_1000_autoReadGetAndSet_sync=0
       - MAX_ALLOCS_ALLOWED_1000_getHandlers=9050
-      - MAX_ALLOCS_ALLOWED_1000_getHandlers_sync=36
+      - MAX_ALLOCS_ALLOWED_1000_getHandlers_sync=38
       - MAX_ALLOCS_ALLOWED_1000_reqs_1_conn=30450
       - MAX_ALLOCS_ALLOWED_1000_tcpbootstraps=4050
       - MAX_ALLOCS_ALLOWED_1000_tcpconnections=165050
-      - MAX_ALLOCS_ALLOWED_1000_udp_reqs=12050
+      - MAX_ALLOCS_ALLOWED_1000_udp_reqs=12000
       - MAX_ALLOCS_ALLOWED_1000_udpbootstraps=2050
       - MAX_ALLOCS_ALLOWED_1000_udpconnections=89950
       - MAX_ALLOCS_ALLOWED_1_reqs_1000_conn=431000

docker/docker-compose.2004.55.yaml

@@ -18,21 +18,21 @@ services:
   test:
     image: swift-nio:20.04-5.5
     environment:
-      - MAX_ALLOCS_ALLOWED_1000_addHandlers=46050
-      - MAX_ALLOCS_ALLOWED_1000_addHandlers_sync=39050
+      - MAX_ALLOCS_ALLOWED_1000_addHandlers=48050
+      - MAX_ALLOCS_ALLOWED_1000_addHandlers_sync=41050
       - MAX_ALLOCS_ALLOWED_1000_addRemoveHandlers_handlercontext=9050
       - MAX_ALLOCS_ALLOWED_1000_addRemoveHandlers_handlername=9050
       - MAX_ALLOCS_ALLOWED_1000_addRemoveHandlers_handlertype=9050
       - MAX_ALLOCS_ALLOWED_1000_autoReadGetAndSet=25050
       - MAX_ALLOCS_ALLOWED_1000_autoReadGetAndSet_sync=0
       - MAX_ALLOCS_ALLOWED_1000_getHandlers=9050
-      - MAX_ALLOCS_ALLOWED_1000_getHandlers_sync=36
+      - MAX_ALLOCS_ALLOWED_1000_getHandlers_sync=38
       - MAX_ALLOCS_ALLOWED_1000_reqs_1_conn=30450
       - MAX_ALLOCS_ALLOWED_1000_tcpbootstraps=4050
       - MAX_ALLOCS_ALLOWED_1000_tcpconnections=165050
       - MAX_ALLOCS_ALLOWED_1000_udp_reqs=12050
       - MAX_ALLOCS_ALLOWED_1000_udpbootstraps=2050
-      - MAX_ALLOCS_ALLOWED_1000_udpconnections=90000
+      - MAX_ALLOCS_ALLOWED_1000_udpconnections=89850
       - MAX_ALLOCS_ALLOWED_1_reqs_1000_conn=431000
       - MAX_ALLOCS_ALLOWED_bytebuffer_lots_of_rw=2050
       - MAX_ALLOCS_ALLOWED_creating_10000_headers=0

docker/docker-compose.2004.main.yaml

@@ -18,15 +18,15 @@ services:
   test:
     image: swift-nio:20.04-main
     environment:
-      - MAX_ALLOCS_ALLOWED_1000_addHandlers=46050
-      - MAX_ALLOCS_ALLOWED_1000_addHandlers_sync=39050
+      - MAX_ALLOCS_ALLOWED_1000_addHandlers=48050
+      - MAX_ALLOCS_ALLOWED_1000_addHandlers_sync=41050
       - MAX_ALLOCS_ALLOWED_1000_addRemoveHandlers_handlercontext=9050
       - MAX_ALLOCS_ALLOWED_1000_addRemoveHandlers_handlername=9050
       - MAX_ALLOCS_ALLOWED_1000_addRemoveHandlers_handlertype=9050
       - MAX_ALLOCS_ALLOWED_1000_autoReadGetAndSet=24050
       - MAX_ALLOCS_ALLOWED_1000_autoReadGetAndSet_sync=0
       - MAX_ALLOCS_ALLOWED_1000_getHandlers=9050
-      - MAX_ALLOCS_ALLOWED_1000_getHandlers_sync=36
+      - MAX_ALLOCS_ALLOWED_1000_getHandlers_sync=38
       - MAX_ALLOCS_ALLOWED_1000_reqs_1_conn=30400
       - MAX_ALLOCS_ALLOWED_1000_tcpbootstraps=4050
       - MAX_ALLOCS_ALLOWED_1000_tcpconnections=161050