Peter M. Groen / FlashMQ

The extraneous creation would happen when multiple subscribes at
different QoS levels happen.

Also add a test to test all scenarios.

This also fixes session clean_start, which was never done since the
MQTT5 refactor to clean_start (vs clean session).

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.

I need to decide what to do with getPublishData

and that disabled test needs repurposing.

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.