Log warning when fetching events stream ahead of change cache

If this is ever triggered there is potential to miss events that have
just been persisted because the change cache is not up to date.

synapse/storage/databases/main/stream.py

@@ -468,6 +468,15 @@ async def get_room_events_stream_for_rooms(
                 - list of recent events in the room
                 - stream ordering key for the start of the chunk of events returned.
         """
+
+        if to_key.stream > self._events_stream_cache.max_stream_pos:
+            logger.warning(
+                "Selecting from room events stream using position %s ahead of "
+                "the stream cache position %s",
+                to_key.stream,
+                self._events_stream_cache.max_stream_pos,
+            )
+
         room_ids = self._events_stream_cache.get_entities_changed(
             room_ids, from_key.stream
         )