Directly lookup local membership instead of getting all members in a room first (get_users_in_room mis-use) (#13608)

See https://github.com/matrix-org/synapse/pull/13575#discussion_r953023755

synapse/storage/databases/main/roommember.py

@@ -534,6 +534,32 @@ class RoomMemberWorkerStore(EventsWorkerStore):
             desc="get_local_users_in_room",
         )
 
+    async def check_local_user_in_room(self, user_id: str, room_id: str) -> bool:
+        """
+        Check whether a given local user is currently joined to the given room.
+
+        Returns:
+            A boolean indicating whether the user is currently joined to the room
+
+        Raises:
+            Exeption when called with a non-local user to this homeserver
+        """
+        if not self.hs.is_mine_id(user_id):
+            raise Exception(
+                "Cannot call 'check_local_user_in_room' on "
+                "non-local user %s" % (user_id,),
+            )
+
+        (
+            membership,
+            member_event_id,
+        ) = await self.get_local_current_membership_for_user_in_room(
+            user_id=user_id,
+            room_id=room_id,
+        )
+
+        return membership == Membership.JOIN
+
     async def get_local_current_membership_for_user_in_room(
         self, user_id: str, room_id: str
     ) -> Tuple[Optional[str], Optional[str]]: