IsDaemon.save_state could be more robust

[untzag]

• save_state triggers only when a setter of state is called (consult the State class and it's updated property). This trigger will not be set if an element of a list in state is altered (e.g. self._state["my_list"][3] = val").
• save_state could be incorporated in the IsDaemon.close method, to ensure state is saved at shutdown

I would do it in shutdown, not close (because many daemons that have a close do not call super because it is a no-op)

We actually do explicitly save the state in shutdown_all, but that is only used on POSIX systems, not windows...