jobs: allow to pause/resume/send at back of queue an executable job

[tahini]

There are many use cases to pause jobs and have them resume, automatically or manually:

• Free resources in a local setup
• Pause a long job to let a smaller one run
• Let a job create other jobs and wait to execute until they are all finished

In all cases, the paused job should suspend its execution and free the worker thread. The operation is similar to cancel, except that it will be resumed later, so it needs to be able to do so. It is more like shutting down the server actually and resuming when starting the server again.

To support pausing, a job needs to implement proper checkpointing, similar to the TrRoutingBatch job.

The UI's executable job list component should also support pausing jobs. One can pause one or many jobs at once and each could be resumed individually or all at once. Only pending and inProgress jobs can be paused.

[tahini]

I started implementing this, it works well.

But I have a philosophical question about "pausing":

Do we need a different status/workflow for "pausing" vs "cancelled"? Or is the difference more semantically in the human mind, but those 2 statuses behave the same?

For manual pause/cancellation, I suppose we could resume tasks with either status. But for automatic task pausing/resuming, like a job with dependencies that would "pause" while they execute, then it would be 2 semantically different statuses: we can automatically resume a paused task if pre-conditions are met, but we cannot automatically resume a cancelled task, only manually.

Does this distinction make sense?