Peter M. Groen / FlashMQ

Also include a few stats.

Unsubscribing paths that didn't exist caused creation of null nodes,
which subsequent use of the tree crashed on.

My IDE didn't understand them for finding symbols, apparently.

Also changed the disconnect reason a little: it now shows them all, so I
don't have to worry about which one is more important.

This includes a timer mechanism.

Connected to this is preventing duplicate subscriptions. It's a bit
unclear what to do when you get a subscription for the same topic with a
different QoS? Change the Qos? Ignore?

Also includes fixes to packet parsing that I couldn't make a separate
commit for.

When it comes to QoS 1, these things are still left, off the top of my
head:

- vector for qos queue? It helps with ordering and is CPU cache friendly.
- Store subscription QoS.
- Do retained messages have QoS?
- Give session client's name, to access it later.

In preparation for clean session and qos.

This saves mildly costly calls to find().

It's a little faster.

Also materializes some concepts about MqttPacket.

Roughly...