Use state_key rather than target_user_id

synapse/handlers/federation.py

@@ -114,7 +114,7 @@ class FederationHandler(BaseHandler):
 
         if event.type == RoomMemberEvent.TYPE:
             if event.membership == Membership.JOIN:
-                user = self.hs.parse_userid(event.target_user_id)
+                user = self.hs.parse_userid(event.state_key)
                 self.distributor.fire(
                     "user_joined_room", user=user, room_id=event.room_id
                 )