Workers can be soft-deleted, which means that they stay in the database.
As such, foreign key constraints `ON DELETE CASCADE` do not trigger, and
thus their sleep schedule can still be active. This is now detected and
handled gracefully.
When the Manager was shutting down while the sleep scheduler was running, it
could cause a null pointer dereference. This is now doubly solved:
- `worker.Identifier()` is now nil-safe, as in, `worker` can be `nil` and
it will still return a sensible string.
- failure to apply the sleep schedule due to the context closing is not
logged as error any more.
