Peter M. Groen / FlashMQ

Only split when they're needed, instead of pre-determining whether I'll
need the subtopics.

This makes the coming refactor of authentication easier.

Also treated user properties with the same brush a bit. Downside: it now
always assigns user properties to newly constructed publish objects,
even if they are not needed (because the generated packet may only be
needed for writing to the client's output buffer). But determining the
flow of when and when not they'll be needed is impossible with the
coming authentication refactor.

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

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.