Implement a Persistent Job Scheduler

[alexanderkiel]

For issues like #1442 we need a persistent job scheduler. We already have a scheduler module.

The jobs are used for the following scenarios:

• (re)building indices
• index cleanup
• calculating statistics
• calculating bloom filters

All jobs have the following properties:

• there execution is idempotent so that they can be rerun partially

What the jobs can't do:

• non-idempotent tasks like sending messages only once

The new functionality should provide:

• persistent job states
  • job's that run longer should survive restarts
  • a job should be able to persist it's state regularly to be able to continue after a restart
  • jobs states should be persisted in reasonably long intervals like minutes not milliseconds to not to overwhelm storage
  • the idea is to have one column family were the key is the job id and the value is the job state
  • a type to be able to execute the right code
  • an status of ready, cancelled, in-progress, failed and completed
• a multi-method that provides the implementation of each job type