Acknowledge item as successfully consumed.

        Removes uid from ackbuffer and deletes the corresponding item.

def ack(self, uid):
        """Acknowledge item as successfully consumed.

        Removes uid from ackbuffer and deletes the corresponding item.
        """
        self._redis.pipeline()\
                   .lrem(self.ACKBUF_KEY, 0, uid)\
                   .lrem(self.BACKUP, 0, uid)\
                   .hdel(self.ITEMS_KEY, uid)\
                   .execute()