Chaining unfulfilled handled Promises must share the handler

[michaelfig]

When returning a promise from an eventual send, if the target of the send was unfulfilled and had an unfulfilled handler, the result should share the same unfulfilled handler until it is fulfilled. Thus the result must also be a handled promise, not an unhandled promise.

This allows the unfulfilled handler to be the coordinator for all promise chains involving the same logical destination, and thereby to perform pipelining.

If the promise resolves to a presence, the fulfilled handler takes over the task of coordinator.

[warner]

in the old repo. this was eventual-send issue 21