Ensure we mark sent knocks as outliers (#10873)

changelog.d/10873.bugfix

+Fix a bug introduced in Synapse 1.37.0 which caused `knock` events which we sent to remote servers to be incorrectly stored in the local database.

synapse/handlers/federation.py

@@ -593,6 +593,13 @@ class FederationHandler(BaseHandler):
             target_hosts, room_id, knockee, Membership.KNOCK, content, params=params
         )
 
+        # Mark the knock as an outlier as we don't yet have the state at this point in
+        # the DAG.
+        event.internal_metadata.outlier = True
+
+        # ... but tell /sync to send it to clients anyway.
+        event.internal_metadata.out_of_band_membership = True
+
         # Record the room ID and its version so that we have a record of the room
         await self._maybe_store_room_on_outlier_membership(
             room_id=event.room_id, room_version=event_format_version