Peter M. Groen / FlashMQ

This makes much more sense than returning the amount of messages sent
all the way up the call stack.

They don't need object state.

There's no point in keeping a vector per nanosecond.

There's no point in keeping a vector of removals per nanosecond.

This is fast(er).

This is way faster.

It merely drops packets when they exceed it. The specs are unclear about
whether you're supposed to delay transmission until the quota is
non-negative again. I decided against it because of increased
complexity, and because on a continously overloaded client, this makes
no sense.

Effectively, this formalizes the 'max qos pending' mechanism that was
already in place.

It also includes PUBACK/PUBREL/PUBCOMP error handling, because that
needed to be done for proper quota control.

This required a special type WillPublish to make this easier and more
logical.

This is better in line with how the copy factory is meant to be used. It
actually broke on the assert on 'externallyReceived' before.

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.

This replaces the old style checking of expired sessions once every 10
minutes or so.

The behavior for MQTT3 clients in the same, but I replaced the term
'clean session' and described the behavior in MQTT5 terms, of 'clean
start' and an expiry interval.

Most of it is limits we already implemented non-standard compliant.

This entails making copies of the original packet when necessary,
because QoS 0 doesn't have a packet id. I tried to keep it to an
absolute minimum and do some precarious optmizations for it. There are
tests though.

The initial concept of iterating over a vector being fast didn't help,
and made having many subscribers to one topic very slow.

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.