Deadlock when subscribing to many topics

- [x] Bug exists Release Version 1.2.5 ( Master Branch)
- [x] Bug exists in MQTTv3 Client on Snapshot Version 1.2.6-SNAPSHOT (Develop Branch)
- [x] Bug exists in MQTTv5 Client on Snapshot Version 1.2.6-SNAPSHOT (Develop Branch)

If we call subscribe in `MqttCallback.connectComplete(boolean arg0, String arg1)` we get a deadlock when we try to subscribe to more than 5 topics. I don't know if this has to do with my usecase only.

Example to reproduce error

```
public static void main(String[] args) {

String broker = "tcp://localhost:1883";
String clientId = "deadlock";

System.out.println("Starting MQTT Client " + clientId);

try {
MqttConnectOptions connOpts = new MqttConnectOptions();
connOpts.setAutomaticReconnect(true);
connOpts.setCleanSession(true);
MqttClient client = new MqttClient(broker, clientId);
client.setCallback(new MqttCallbackExtended() {

@Override
public void messageArrived(String arg0, MqttMessage arg1) throws Exception {
System.out.println("new msg: " + arg0 + " " + arg1);
}

@Override
public void deliveryComplete(IMqttDeliveryToken arg0) {

}

@Override
public void connectionLost(Throwable arg0) {
System.err.println("Lost connection");
}

@Override
public void connectComplete(boolean arg0, String arg1) {
try {
System.out.println("Subscribing to topics");

client.subscribe("mysys/+/topic0/#", 0);
client.subscribe("mysys/+/topic1", 0);
client.subscribe("mysys/+/topic2/+");
client.subscribe("mysys/+/topic3/+", 0);
client.subscribe("mysys/+/topic4/#", 0);
// client.subscribe("mysys/+/topic5/+", 0); //if we add this subscribe everything locks. None of the subscriptions above will work, no messages arrive.
} catch (MqttException e) {
e.printStackTrace();
}
}
});
client.connect(connOpts);
} catch (MqttException e) {
e.printStackTrace();
}
}
```

Is this the intended behaviour or is it a bug?

To get around this problem you can place all the subscribe actions on a separate thread in your application. Like this.

```
public static void main(String[] args) {

String broker = "tcp://localhost:1883";
String clientId = "deadlock";

System.out.println("Starting MQTT Client " + clientId);

try {
MqttConnectOptions connOpts = new MqttConnectOptions();
connOpts.setAutomaticReconnect(true);
connOpts.setCleanSession(true);
final MqttClient client = new MqttClient(broker, clientId);
client.setCallback(new MqttCallbackExtended() {

@Override
public void messageArrived(String arg0, MqttMessage arg1) throws Exception {
System.out.println("new msg: " + arg0 + " " + arg1);
}

@Override
public void deliveryComplete(IMqttDeliveryToken arg0) {

}

@Override
public void connectionLost(Throwable arg0) {
System.err.println("Lost connection");
}

@Override
public void connectComplete(boolean arg0, String arg1) {
try {
System.out.println("Subscribing to topics");
new Thread(new Runnable() {

@Override
public void run() {
try {
client.subscribe("mysys/+/topic0/#", 0);
client.subscribe("mysys/+/topic1", 0);
client.subscribe("mysys/+/topic2/+");
client.subscribe("mysys/+/topic3/+", 0);
client.subscribe("mysys/+/topic4/#", 0);
client.subscribe("mysys/+/topic5/+", 0); // this works now as well and everything subscribes.
} catch (MqttException e) {
e.printStackTrace();
}

}
}).start();
} catch (Exception e) {
e.printStackTrace();
}
}
});
client.connect(connOpts);
} catch (MqttException e) {
e.printStackTrace();
}
}
```