Allow jobs to run in different process classes

Current Situation

• With JS-1301 Job chains can have default process classes.
• If a Job is used in different Job chains with different process classes, JobScheduler might raise an error if a task for this job is already running but needs to be started for another process class.

Desired Behaviour

• A job should be able to run in different process classes
  • at the same time (if enough tasks are available)
  • sequentially (if it needs to be started for more process classes than available tasks)

Limitations

• Jobs that are assigned a process class via a job chain do not cause idle jobs to end themselves in order to release their slot in the process class.
• This limitation is addressed by JS-1481

Considerations
under construction

• The tasks attribute of the job specifies the allowed number of tasks of this job per JobScheduler Master, not per process_class.
  • If a job has tasks="3" and is configured for 2 process classes, then a total number of 3 instances of this job is allowed, not 6.
• If tasks are running and a task start is requested for a process class for which no task is running or no running task is idle and...
  • if there are still tasks available for the job, then a new task is started for that process class
  • if there are no tasks available for the job and...
    • a task of another process class is idle, then the idle task is ended in favor of the waiting process class
    • all other tasks of this job are busy, then then the order has to wait
      • do we need some algorithm here (round-robin) to prevent starving?