Peter M. Groen / FlashMQ

This fixes not handling the last packet(s) of a client that disconnects.

Because that's what's it is now.

A lot of code can be refactored to get the settings from this now, but
I'm not going to do that yet.

There was not really a benefit to the old method, and this prevents
packet pile up and keeps the code cleaner.

One fix is client destruction happening on the correct thread (when
kicking another one off with existing client ID). This caused deadlocks
on the subscriptions lock during a race condition when
doKeepAliveCheck() also ran.

A related deadlock was that the queued functions were executed while
holding the lock taskQueueMutex. Together with the subscriptions lock,
that was executed in the $SYS topic function, this also caused
deadlocks.