Add fast path for replication events stream fetch (#16580)

We can bail early if the from token is greater than or equal to the
current token.

Author: erikjohnston

changelog.d/16580.bugfix

@@ -0,0 +1 @@
+Fix a long-standing, exceedingly rare edge case where the first event persisted by a new event persister worker might not be sent down `/sync`.

synapse/replication/tcp/streams/events.py

@@ -157,6 +157,12 @@ async def _update_function(
         current_token: Token,
         target_row_count: int,
     ) -> StreamUpdateResult:
+        # The events stream cannot be "reset", so its safe to return early if
+        # the from token is larger than the current token (the DB query will
+        # trivially return 0 rows anyway).
+        if from_token >= current_token:
+            return [], current_token, False
+
         # the events stream merges together three separate sources:
         #  * new events
         #  * current_state changes