dnp3: fixups to work with unified json tx logger

dnp3: fixups to work with unified json tx logger

Update DNP3 to work with a single TX logger, and just register one
logger instead of 2.

This primarily creates a TX per message instead of correlating replies
to requests, which fits the DNP3 model better, but we didn't really have
this concept nailed down when DNP3 was written.