Assorted tweaks on further review and local testing:

Assorted tweaks on further review and local testing:
* Default 1 child process
* Introduce a log system so that critical errors can be filtered out of the progress messages
* Warn on abnormal child termination
* Use the actual row count for progress instead of the end ID. I think we can afford to count rows.
* Check for bt_moved=0 in doOrphanList() after finishIncompleteMoves(), to avoid attempted double-move and resulting spurious "conflict detected" warning