Add per-app flock locking around deploy, config change, rollback, new-app

Nothing previously serialized these against each other, so concurrent
invocations for the same app (a manual config change racing an
in-flight deploy, a double git push, a rollback mid-deploy) could
interleave snapshot/apply/restart/rollback sequences against the same
dataset and jail. The lock file lives under a new LocksDir, outside
any per-app ZFS dataset, since those datasets get rolled back on a
failed health check or a manual rollback.

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