Schedule service on_quit cancels pending scheduled events

Currently, `on_quit` does not attempt to handle the greenlets started with `spawn_later` that is the core of the SchedulerService.  This patch attempts to fix that, with respect to the possible coming restart.

I notice there is an `on_system_restart` but I am not sure where this is called from - maybe a bootstrap service call?  That would make the most sense.

This PR for review please.