Peter M. Groen / FlashMQ

I think it's correct, but mosquitto_sub doesn't seem to support it, so I
can't test.

Also some other stuff I happen to see.

And one line about correlation data that was a bug.

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

I need to decide what to do with getPublishData

and that disabled test needs repurposing.

which also has implications on how I think to handle 'queuePacketAtSub'

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.

Especially remove the flashmq_auth_plugin_allocate_global_memory(),
which was an old idea.

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).

For the coming MQTT5 support, I'll need this a lot.

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.

This doesn't actually fix a real bug, just makes assumptions clear.

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

This is an (insignificant) amount slower, but otherwise existing
sessions won't get the new limits when reloading the config.