Overhaul cron-notify idle

[PhrozenByte]

• If registering the sleep/resume callback is successful, don't wake up once a hour to check the state (this is a borg-notify leftover where it was possible to run multiple instances at the same time)
  • Open question: What should we do if registering the sleep/resume callback isn't possible?
• Never sleep longer (CronNotify._sleepTime) than the next execution time (i.e. when cron = * * * * * and sleepTime = 3600, you must never sleep longer than 60 sec, depending on the current time)
• Improve class interface to have definite resetTimer and clearTimer methods per timer (next execution timer, notification timeout timer, dismiss timer)