threads: refactor TmThreadsSlotPktAcqLoop for user threads

threads: refactor TmThreadsSlotPktAcqLoop for user threads

Refactor TmThreadsSlotPktAcqLoop for user provided thread by breaking
out the init and finish code into their own functions.

For user provided threads, Suricata should not "drive" the thread, but
the setup and finish code is the same.

The finish function is exported so it can be called by the user
application when its receive loop or equivalent is done.

Also remove obsolete comment.

Ticket: #7240