Don't abort a batch whose items were all processed (processingCompletedAt set)
before the seal landed; those runs resume the parent on their own.
Keep expireBatch idempotent: an already-aborted batch still resolves its parent
waitpoint, so a retry after a mid-run crash can't leave the parent blocked.
Treat ABORTED as terminal in tryCompleteBatch so a straggler run finalizing
can't flip the batch back to COMPLETED.
Abort the batch if scheduling the reaper fails, so a blocked parent is never
stranded with nothing to free it.
Matt Aitken
79bd92e17e