Write next packet in send queue.

def send_packet(self):
        """Write next packet in send queue."""
        waiter, packet = self.client.waiters.popleft()
        self.logger.debug('sending packet: %s', binascii.hexlify(packet))
        self.client.active_transaction = waiter
        self.client.in_transaction = True
        self.client.active_packet = packet
        self.reset_cmd_timeout()
        self.transport.write(packet)