Handles pong results, which don't come inside a ``rpc_result``
        but are still sent through a request:

            pong#347773c5 msg_id:long ping_id:long = Pong;

async def _handle_pong(self, message):
        """
        Handles pong results, which don't come inside a ``rpc_result``
        but are still sent through a request:

            pong#347773c5 msg_id:long ping_id:long = Pong;
        """
        pong = message.obj
        self._log.debug('Handling pong for message %d', pong.msg_id)
        state = self._pending_state.pop(pong.msg_id, None)
        if state:
            state.future.set_result(pong)