Re-implement Scheduler without Quartz

Quartz is problematic in cluster mode for several reasons:

• the SQL initialization of the needed tables is not automatic or even documented clearly,
• the job persistence is way too complex for our needs and often causes locking issues
• we need additional features like the possibility to execute scheduled jobs on all cluster nodes, not just a single one.

-> Replace it with a custom implementation, and use Redis for the needed node coordination in cluster mode.