Peter M. Groen / FlashMQ

This allows easier saving of MQTT5 properties, for which a new file
version for retained messages is created. It uses the packet parsing logic.

- Delay is properly counted from moment of disconnect.
- When a session is picked up again, the will is not sent.
- An actual fix to what I tried a few commits ago: fix sending will
  twice.

This logic should make storing wills also easier.

The loop of pending messages was stuck.

We saved a session expiry interval of 0. I may need to change that
session copying idea.

Also fixes a bug in new interval calculation.

And the TODOs are for the next thing: saving the createdAt date.

This prevents bugs because the calling context forgets it. A (small)
downside is that I have to make the Publish argument non-const. But,
that's exactly what it is then, so...

Because clients can now also exist as dummy objects, I had to add some
extra checks.

Also split up handlePublish() and the new parsePublishData().

I'm not sure how I'm continueing. I need a safe point.

But, this is a safe point before I will refactor it. I will remove the
appStartTime and session last touched. With the new queued removals,
this is no longer necessary.

This saves some allocations.

This also meant having to set the C++ standard to 2014.

The getCopy() methods of sessions and mqttpackets can't be changed,
because of access errors (private).

This is a preparation for MQTT5, because when there are receivers and
publishers with different protocols, you can't always just write out the
same packet. You can sometimes though, so that's what the copy factory
determines.

When there are many sessions, the copying action can actually take some
time, and memory. A vector is faster and uses (a little) less memory.

This is a theoretical fix, without benchmarks to support it.

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.