Generate a unique identifier for an ACK packet.

def _generate_ack_id(self, sid, namespace, callback):
        """Generate a unique identifier for an ACK packet."""
        namespace = namespace or '/'
        if sid not in self.callbacks:
            self.callbacks[sid] = {}
        if namespace not in self.callbacks[sid]:
            self.callbacks[sid][namespace] = {0: itertools.count(1)}
        id = six.next(self.callbacks[sid][namespace][0])
        self.callbacks[sid][namespace][id] = callback
        return id