A backup step killed by a time limit, OOM or kill -9 kept Nextcloud's
lock on its location for up to an hour, blocking every new start and the
run itself. Locations and runs are now guarded by leases in ngb_leases
with an expiry: a backup step holds them for its duration plus two
minutes, restores refresh theirs every minute, prune holds 30 minutes,
untrash 5. Nextcloud's lock is only used for the milliseconds in which a
lease is checked and written.
Also: starting a backup while one is already running simply continues
it (no exclusive lease needed), and running to completion waits instead
of spinning while a crashed step's lease has not expired.
Tested on NC34: a backup killed four times at random moments resumes on
its own after the lease expires (about two minutes), and restores
bit-identically.
