activate and stop schedules in scheduler xml

New command:
<schedule.modify name="my_schedule" state="stopped"/>
<schedule.modify name="my_schedule" state="running"/>
<schedule.modify name="my_schedule" state="stopped" temporary="yes"/>

In XML answer:
<schedule name="my_schedule" state="stopped"/>

Schedule states need to be persistent over Scheduler restarts --> store in db (unless temporary="yes", default is "no")

This must be implemented in the same way as stopping a job chain node or a job.