Fix call to have_seen_events

Let's actually implement `have_seen_event`, to make it easier for people to not
shoot their feet off here.

synapse/handlers/federation_event.py

@@ -989,7 +989,7 @@ async def _get_state_and_persist(
         )
 
         # we also need the event itself.
-        if not await self._store.have_seen_events(room_id, event_id):
+        if not await self._store.have_seen_event(room_id, event_id):
             await self._get_events_and_persist(
                 destination=destination, room_id=room_id, event_ids=(event_id,)
             )

synapse/storage/databases/main/events_worker.py

@@ -22,7 +22,6 @@
     Dict,
     Iterable,
     List,
-    NoReturn,
     Optional,
     Set,
     Tuple,
@@ -1330,10 +1329,9 @@ def have_seen_events_txn(
         return results
 
     @cached(max_entries=100000, tree=True)
-    async def have_seen_event(self, room_id: str, event_id: str) -> NoReturn:
-        # this only exists for the benefit of the @cachedList descriptor on
-        # _have_seen_events_dict
-        raise NotImplementedError()
+    async def have_seen_event(self, room_id: str, event_id: str) -> bool:
+        res = await self._have_seen_events_dict(((room_id, event_id),))
+        return res[(room_id, event_id)]
 
     def _get_current_state_event_counts_txn(
         self, txn: LoggingTransaction, room_id: str