Lingering thread in MqttClient

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- [x ] Bug exists Release Version 1.2.0 ( Master Branch)
- [x ] 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)

I am developing a tomcat servlet that exchanges information via Mqtt. Tomcat 8, java 1.8.

I create a client in the HttpServlet init() method:
```java
if (myClient == null) {
try {
myClient = new MqttClient("tcp://localhost:1883", MqttClient.generateClientId(), persistence);
MqttConnectOptions connOpts = new MqttConnectOptions();
connOpts.setCleanSession(true);
connOpts.setUserName("myusername");
connOpts.setPassword("mypassword".toCharArray());
connOpts.setConnectionTimeout(300);
connOpts.setKeepAliveInterval(30);
connOpts.setMqttVersion(MqttConnectOptions.MQTT_VERSION_3_1_1);
myClient.connect(connOpts);
getServletContext().setAttribute("myClient", myClient);
} catch (MqttException e) {
this.log.error ("MqttException:\n" + e.getMessage());
}
}
```

I subscribe to a message:
```java
receivedSignal = new CountDownLatch(1);
myClient.subscribe(patient_id, (topic, msg) -> {
String payload = new String(msg.getPayload());
this.log.info("Got message: " + payload);
myObjectBuilder.add("result", "good");
myObjectBuilder.add("payload", payload);
receivedSignal.countDown();
});
```

In the HttpServlet destroy() method:

```java
System.out.println("destroy");
if (receivedSignal!= null) {
System.out.println("destroy CountDownLatch: " + receivedSignal.toString());
receivedSignal.countDown();
}
if (persistence!=null) {
try {
persistence.close();
System.out.println("persistence closed in destroy");
} catch (MqttPersistenceException e) {
System.out.println ("MqttPersistenceException:\n" + e.getMessage());
}
}
if (myClient != null) {
try {
myClient.disconnect();
myClient.disconnectForcibly();
myClient.close(true);
myClient = null;
System.out.println("myClient closed in destroy");
} catch (MqttException e) {
System.out.println ("MqttException:\n" + e.getMessage());
}
}
super.destroy();
```

This works. The problem is that whenever I update the war, I get this in my log:

```
destroy
persistence closed in destroy
myClient closed in destroy
14-Jun-2019 10:58:19.836 WARNING [ContainerBackgroundProcessor[StandardEngine[Catalina]]] org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesThreads The web application [main] appears to have started a thread named [nioEventLoopGroup-2-1] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
sun.nio.ch.KQueueArrayWrapper.kevent0(Native Method)
sun.nio.ch.KQueueArrayWrapper.poll(KQueueArrayWrapper.java:198)
sun.nio.ch.KQueueSelectorImpl.doSelect(KQueueSelectorImpl.java:117)
sun.nio.ch.SelectorImpl.lockAndDoSelect(SelectorImpl.java:86)
sun.nio.ch.SelectorImpl.select(SelectorImpl.java:97)
io.netty.channel.nio.SelectedSelectionKeySetSelector.select(SelectedSelectionKeySetSelector.java:62)
io.netty.channel.nio.NioEventLoop.select(NioEventLoop.java:791)
io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:439)
io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:906)
io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
java.lang.Thread.run(Thread.java:748)
```

I've increased the Context unloadDelay to 10 seconds, but the thread is still there. If I restart tomcat, the thread dies (as best I can tell). If I don't, eventually tomcat hangs due to the open threads. This shouldn't be an issue in production, but during development, it is highly annoying. It happens in both org.eclipse.paho.client.mqttv3-1.2.1 and in org.eclipse.paho.client.mqttv3-1.2.2-20190410.135359-1. I've seen some discussion of this in previous issues, but it seems like the threads should shut down when .close() is called.