Better handle interaction of update windows and check interval

[gibmat]

Currently it's possible to define an update window (say, for example daily from 2am - 4am) and the default check interval of six hours will result in IncusOS never actually being able to check for an update except if you happen to have initially booted the system sometime between 2am - 4am.

We should process maintenance windows before sleeping, and then sleep the minimum time of the configured check interval and the next maintenance window (if any).