Scheduler and Job classes refactoring

Context

In order in ease the integration of new features like job cancellation of scheduler instrumentation, the Job and Scheduler class would benefit from a refactoring.

Task Description

• Refactor the Job class for better encapsulation and new public interface:
  • Job.register_client() and Job.unregister_client(): add/remove an operation watcher.
  • Job.set_cached_result(): manually associate a result that comes from the action cache.
  • Job.create_lease() and Job.update_lease_state(): lease state manipulation API for the scheduler.
  • Job.cancel_lease(): lease cancellation call.
  • Job.update_operation_stage(): operation stage manipulation API for the scheduler.
  • Job.cancel_operation(): operation cancellation call.
• Refactor the Scheduler with new public interface:
  • Scheduler.register_client() and Scheduler.unregister_client(): add/remove a job's operation watcher.
  • Scheduler.queue_job() and Scheduler.retry_job(): job handling API for the execution/operation interface.
  • Scheduler.request_job_leases() and Scheduler.update_job_lease_state(): update a job's lease state from the bot interface.
  • Scheduler.cancel_operation(): operation cancellation for the operation interface.

(Cancellation is out of the scope of that issue, see #48 (closed) and #74 (closed))