Low: scheduler: respect shutdown locks when placing inactive resources

When shutdown-lock is enabled, and we're either scheduling a resource stop
on a node that's cleanly shutting down or scheduling any action for a
previously locked resource, add "shutdown-lock=<shutdown-timestamp>" to the
graph action. The controller will be able to use this to know when to preserve
the lock (by adding the lock time to the resource state entry).

When the scheduler unpacks a resource state entry with a lock, it will remember
the lock node and lock time, which will trigger existing code for applying
shutdown locks.