Reintroduce CHECKPOINT as a clever persist point

Currently persists are often used to checkpoint progress through a workunit - but also allow the ecl to be modified and the job resubmitted.

The problem with that system is the persists are not automatically deleted when they are no longer needed.

CHECKPOINT would create persist points which were also mangled with the job name. The checkpoints would be deleted when they were no longer needed - i.e. when all other checkpoints have read the checkpoint, or when the entire job is completed.

If the job never succeeds they would be deleted when they expire.