Peter M. Groen / FlashMQ

The only mutable session data of a client is QoS related, so when we're
copying sessions (for saving them), we need to lock the QoS data,
because that gets modified from active client traffic in worker threads.

Note: not super well tested at this point, nor was I ever able to
trigger actual errors despite long stress testing, so it's a theoretical
fix.

This method incurs no extra CPU load when messages aren't dropped.

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.

There were bugs in which authentication object was used when, causing
threadings bugs. Instead of getting from the 'sender', we can just store
a thread local pointer.

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.

Also include a few stats.

Also fixes not downgrading QoS on subscribe.

Encrypted version only.

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

And check Qos.

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.