Wait for streams to catch up when processing HTTP replication. (#14820)

This should hopefully mitigate a class of races where data gets out of
sync due a HTTP replication request racing with the replication streams.

synapse/handlers/federation_event.py

@@ -2259,6 +2259,10 @@ class FederationEventHandler:
                 event_and_contexts, backfilled=backfilled
             )
 
+            # After persistence we always need to notify replication there may
+            # be new data.
+            self._notifier.notify_replication()
+
             if self._ephemeral_messages_enabled:
                 for event in events:
                     # If there's an expiry timestamp on the event, schedule its expiry.