Adds stop and resume methods to Scheduler implementations.
Previously, "ending" a scheduler would simply delete the schedule. By adding a separate "stop" method, we can pause and resume schedules. In the case of the SystemCronScheduler:
- start creates metadata files (.json and .sh) and creates a cron tab
- stop simply removes the cron tab
- resume puts the cron tab back
- end deletes all metadata files and removes the crontab