Spawn an async request to a remote webserver.

def async_send(self, url, data, headers, success_cb, failure_cb):
        """
        Spawn an async request to a remote webserver.
        """
        # this can be optimized by making a custom self.send that does not
        # read the response since we don't use it.
        self._lock.acquire()
        return gevent.spawn(
            super(GeventedHTTPTransport, self).send, url, data, headers
        ).link(lambda x: self._done(x, success_cb, failure_cb))