Disconnecting/closing client not possible after connection failed

- [x] Bug exists Release Version 1.2.0 ( Master Branch)
- [ ] Bug exists in MQTTv3 Client on Snapshot Version 1.2.1-SNAPSHOT (Develop Branch)
- [ ] Bug exists in MQTTv5 Client on Snapshot Version 1.2.1-SNAPSHOT (Develop Branch)

Hi,
we are using the Paho MQTT Java client in the [Eclispe Ditto](https://github.com/eclipse/ditto) project to allow a user to configure MQTT connections to external brokers. That means we do not know in advance if the connections are valid and the user may need some iterations to get all parameters right.
The problem that we now face is that we have serious problems to cleanup the resources used by the Paho Client in case a connection failed. To reproduce the behavior I used the following snippet as "MQTT server":
```java
public static void main(String... args) throws Exception {
final ServerSocket serverSocket = new ServerSocket(4712);
while (true) {
final Socket clientSocket = serverSocket.accept();
System.out.println("Connected: " + clientSocket.getRemoteSocketAddress());
}
}
```
Then I tried to create a connection:
```java
public static void main(String... args) throws Exception {
MqttClient client = new MqttClient("tcp://localhost:4712", "test");
client.setTimeToWait(5000);
try {
MqttConnectOptions connOpts = new MqttConnectOptions();
connOpts.setConnectionTimeout(5);
client.connect(connOpts);
} catch (MqttException me) {
me.printStackTrace();
try {
client.disconnect();
// client.disconnectForcibly(5000); (1)
// client.disconnectForcibly(5000, 5000); (2)
// client.disconnectForcibly(5000, 5000, false); (3)
client.close();
// client.close(true); (4)
} catch (Exception e) {
e.printStackTrace();
}
}
}
```
The main thread is now waiting indefinitely for a response(?) in the disconnect :
```
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x0000000717c00728> (a java.lang.Object)
at java.lang.Object.wait(Object.java:502)
at org.eclipse.paho.client.mqttv3.internal.Token.waitForResponse(Token.java:143)
- locked <0x0000000717c00728> (a java.lang.Object)
at org.eclipse.paho.client.mqttv3.internal.Token.waitForCompletion(Token.java:108)
at org.eclipse.paho.client.mqttv3.MqttToken.waitForCompletion(MqttToken.java:63)
at org.eclipse.paho.client.mqttv3.MqttClient.disconnect(MqttClient.java:349)
at test.PahoTestClient.main(PahoTestClient.java:33)
```
I also tried (1), (2), (3), (4) with the result that the client hangs at a different line, but is also not terminating correctly. Skipping the disconnect and force closing the client does not work either, the client says `Connect already in progress (32110)` in this case.

Currently I see no way of cleaning up after a failed connection attempt. This leaves us with 5 lingering threads which can pile up quickly. Did I miss something or am I doing something wrong here?