Internal Refactoring of Scheduling

The internal handling has been refactored onto gen_stage.
There were a few Breaking Changes which should not influence a user of the library.

Changed

• Replaced call with cast
  • Scheduler.add_job
  • Scheduler.deactivate_job
  • Scheduler.activate_job
  • Scheduler.delete_job
  • Scheduler.delete_all_jobs

Removed

• The overlap handling is removed from the Job struct.
  • removed Job.pids
  • removed Job.executable?