Add timestamp to lock to allow automatically removing stale locks

If a backup crashes the lock is still there, and you have to manually run "--unlock" to rmeove it. It would be better if the lock had a timestamp so Somersault could automatically remove a previous run's lock if it was stale.

The hard part about this is not checking the lock but periodically updating the lock. (That doesn't seem too hard, but its the main change needed.)