Faster joins: persist to database (#12012)

When we get a partial_state response from send_join, store information in the
database about it:
 * store a record about the room as a whole having partial state, and stash the
   list of member servers too.
 * flag the join event itself as having partial state
 * also, for any new events whose prev-events are partial-stated, note that
   they will *also* be partial-stated.

We don't yet make any attempt to interpret this data, so API calls (and a bunch
of other things) are just going to get incorrect data.

synapse/handlers/federation.py

@@ -519,8 +519,17 @@ class FederationHandler:
                 state_events=state,
             )
 
+            if ret.partial_state:
+                await self.store.store_partial_state_room(room_id, ret.servers_in_room)
+
             max_stream_id = await self._federation_event_handler.process_remote_join(
-                origin, room_id, auth_chain, state, event, room_version_obj
+                origin,
+                room_id,
+                auth_chain,
+                state,
+                event,
+                room_version_obj,
+                partial_state=ret.partial_state,
             )
 
             # We wait here until this instance has seen the events come down