nixos/zfs: Refactor auto-snapshots and make them persistent

If you power off your machine frequently, you could miss the execution
of some snapshots.

This is more troublesome the more infrequently the snapshots are
triggered. For example, monthly snapshots only execute at exactly
midnight on the first day of the month.  If you only have your
machine powered on at that time with probability 50%, then half the
snapshots won't be triggered.

This means that if you wanted to keep 3 monthly snapshots, then instead
of keeping 3 months' worth of snapshotted data as you expected, you would
end up with snapshots spanning back 6 months.

Adding the "Persistent = yes" option to auto-snapshot timer units makes
a missed snapshot execute when booting up the machine.

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