MQTTv5 support

[growse]

We should probably support MQTTv5.

In an ideal world, this would be a simple case of updating the MQTT client lib dependency and allowing users to select v5. However, it's not quote that simple. We currently use org.eclipse.paho.client.mqttv3, which supports 3.1 and 3.1.1. Paho has a library that supports v5 (https://github.com/eclipse/paho.mqtt.java/blob/master/MQTTv5.md), but it seems like it only supports v5. So, we'd need to abstract this somehow.

There might be alternative libraries that let us support all the versions, this needs to be investigated.