Fix disconnect bug

GatewayConnection::disconnect did not send the disconnect to the
MqttClient and therefore the MqttClient never disconnected and
therefore the MQTTClient_destroy was called without a
MQTTClient_disconnect which leaves a ‚socket leak‘ behind:

```bash
   sudo lsof -a -p <pid>
   …
   IPv4 TCP gateway:44674->broker:1883 (CLOSE_WAIT)
   …
```