Use stream.current_token() and remove stream_positions() (#7172)

We move the processing of typing and federation replication traffic into their handlers so that `Stream.current_token()` points to a valid token. This allows us to remove `get_streams_to_replicate()` and `stream_positions()`.

synapse/replication/tcp/handler.py

@@ -314,15 +314,7 @@ class ReplicationCommandHandler:
             self._pending_batches.pop(cmd.stream_name, [])
 
             # Find where we previously streamed up to.
-            current_token = self._replication_data_handler.get_streams_to_replicate().get(
-                cmd.stream_name
-            )
-            if current_token is None:
-                logger.warning(
-                    "Got POSITION for stream we're not subscribed to: %s",
-                    cmd.stream_name,
-                )
-                return
+            current_token = stream.current_token()
 
             # If the position token matches our current token then we're up to
             # date and there's nothing to do. Otherwise, fetch all updates