Improve the logging in _auth_and_persist_outliers (#11010)

Include the event ids being peristed

changelog.d/11010.misc

+Clean up some of the federation event authentication code for clarity.

synapse/handlers/federation_event.py

@@ -1161,7 +1161,10 @@ class FederationEventHandler:
                 return
 
             logger.info(
-                "Persisting %i of %i remaining events", len(roots), len(event_map)
+                "Persisting %i of %i remaining outliers: %s",
+                len(roots),
+                len(event_map),
+                shortstr(e.event_id for e in roots),
             )
 
             await self._auth_and_persist_fetched_events_inner(origin, room_id, roots)