MarkUnfinishedOps: update job file on disk

Every time we call MarkUnfinishedOps we do it in a try/finally block
that updates the job file. With this patch we move the try/finally
inside. CancelJobUnlocked is removed, because it just becomes a wrapper
over MarkUnfinishedOps with two constant values.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>