Add API to cancel async jobs

[y5nw]

This PR adds an API to cancel async jobs.

• Goal of the PR
  See above
• How does the PR work?
  The async engine attempts to remove jobs from the queue if it is not yet executed.
• Does it resolve any reported issue?
  Partially addresses No proper way to deal with async job interdependencies / no cancellation #13681
• Does this relate to a goal in the roadmap?
  See linked issue.
• If not a bug fix, why is this PR needed? What usecases does it solve?
  See linked issue.

To do

This PR is a Work in Progress.

• Implement replacing jobs (Dropped)
• Implement job cancellation
• Add tests for
  • Replacing jobs before the job is run (Dropped)
  • Replacing jobs after the job is complete (Dropped)
  • Canceling jobs
• Add documentation

How to test

The new API should be covered by unittests.

Note that I have not yet figured out how to test certain things mentioned in the PR, such as canceling existing async jobs on shutdown.

[sfan5]

some thoughts

[y5nw]

I almost forgot: should this feature also be available for the main menu environment?

[sfan5]

I almost forgot: should this feature also be available for the main menu environment?

No need atm. If someone needs it in the future, they can add it.