Reduce _get_state_group_for_event cache size

synapse/storage/state.py

@@ -507,7 +507,7 @@ class StateStore(SQLBaseStore):
         state_map = yield self.get_state_ids_for_events([event_id], types)
         defer.returnValue(state_map[event_id])
 
-    @cached(num_args=2, max_entries=100000)
+    @cached(num_args=2, max_entries=50000)
     def _get_state_group_for_event(self, room_id, event_id):
         return self._simple_select_one_onecol(
             table="event_to_state_groups",