Improve type checking in replication.tcp.Stream (matrix-org#7291)

The general idea here is to get rid of the type: ignore annotations on all of the current_token and update_function assignments, which would have caught matrix-org#7290.

After a bit of experimentation, it seems like the least-awful way to do this is to pass the offending functions in as parameters to the Stream constructor. Unfortunately that means that the concrete implementations no longer have the same constructor signature as Stream itself, which means that it gets hard to correctly annotate STREAMS_MAP.

I've also introduced a couple of new types, to take out some duplication.

changelog.d/7291.misc

@@ -0,0 +1 @@
+Improve typing annotations in `synapse.replication.tcp.streams.Stream`.

synapse/replication/tcp/streams/__init__.py

@@ -25,8 +25,6 @@
     update_function:    The function that returns a list of updates between two tokens
 """
 
-from typing import Dict, Type
-
 from synapse.replication.tcp.streams._base import (
     AccountDataStream,
     BackfillStream,
@@ -67,8 +65,7 @@
         GroupServerStream,
         UserSignatureStream,
     )
-}  # type: Dict[str, Type[Stream]]
-
+}
 
 __all__ = [
     "STREAMS_MAP",