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/message.py

@@ -992,6 +992,8 @@ class EventCreationHandler:
             and full_state_ids_at_event
             and builder.internal_metadata.is_historical()
         ):
+            # TODO(faster_joins): figure out how this works, and make sure that the
+            #   old state is complete.
             old_state = await self.store.get_events_as_list(full_state_ids_at_event)
             context = await self.state.compute_event_context(event, old_state=old_state)
         else: