Changes:

- In job definitions and apscheduler methods: `job_id` instead of `id`.
- Invoking scheduler.start() within init_app(), because jobs are modified within init_app() (to update possible job
  definition changes to the existing jobstore jobs). Before modifying any jobs the scheduler has to be started.
- REST API extended with reschedule_job, reschedule_job_once and reload_jobs.
