Await un-partial-stating after a partial-state join (#12399)

When we join a room via the faster-joins mechanism, we end up with "partial
state" at some points on the event DAG. Many parts of the codebase need to
wait for the full state to load. So, we implement a mechanism to keep track of
which events have partial state, and wait for them to be fully-populated.

synapse/storage/databases/main/events_worker.py

@@ -1974,7 +1974,15 @@ class EventsWorkerStore(SQLBaseStore):
     async def get_partial_state_events(
         self, event_ids: Collection[str]
     ) -> Dict[str, bool]:
-        """Checks which of the given events have partial state"""
+        """Checks which of the given events have partial state
+
+        Args:
+            event_ids: the events we want to check for partial state.
+
+        Returns:
+            a dict mapping from event id to partial-stateness. We return True for
+            any of the events which are unknown (or are outliers).
+        """
         result = await self.db_pool.simple_select_many_batch(
             table="partial_state_events",
             column="event_id",