Peter M. Groen / FlashMQ

Mandatory normative statement MQTT-3.1.2-6.

One was confirmed: writing an mqtt packet into a client that
disconnected after checking the weak pointer for validity.

The rest made sense to change as well.

Files are simple serialized bytes prefaced by lengths. File is hashed to
verify integrity. This was also a good way preventing unexpected errors
when trying to crash the parser by having it load a different file.

This change includes some refactoring that was necessary:

- It 'fixes' looking at the wrong thread's authentiction. This is still
  wrong though. It will be fixed by a thread local pointer in the next
  commit.
- Deadlocks with yourself are handled in rwlockguard.
- QoSPacketQueue is now a class.
- Probably other tweaks.

This performs a whole lot better.

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...