Flags metadata for update, return Future()

        Actual update must be handled separately. This method will only
        change the reported ttl()

        Returns:
            kafka.future.Future (value will be the cluster object after update)

def request_update(self):
        """Flags metadata for update, return Future()

        Actual update must be handled separately. This method will only
        change the reported ttl()

        Returns:
            kafka.future.Future (value will be the cluster object after update)
        """
        with self._lock:
            self._need_update = True
            if not self._future or self._future.is_done:
              self._future = Future()
            return self._future