Peter M. Groen / FlashMQ

Also fix sending an UNSUBACK instead of SUBACK on unsubscribe.

And also made it test accross protocol versions.

This prevents confusion when another instance of FlashMQ is running.

This is not the final way. For instance, when SSL connections will also
be tested, this won't work anymore.

Now that parsing and handling of packets is separated, we can use the
main code to parse packets in the new FlashMQTestClient. This allows
great flexibility in inspecting the server response in a flexible
manner.

We now also have the ability to make tests for MQTT5 features.