Rework and expand the algorithms in JSCAN, part 2/?.

Rework and expand the algorithms in JSCAN, part 2/?.

* Implement session tracking so multiple output commands can be independantly
  tracked.  However, currently only one output command may be specified
  (e.g. -w or -m, not both, yet)
* The session code now updates transaction id files, giving us a general
  restart capability on the userland side.
* Prefix set record mode (-w) should now work, including journal restarts.
* Running a mirror (-m) on an active prefix set should now operate properly
  in batch mode.

Many things are still not working yet.