Add API to replace an existing async job

[y5nw]

This PR adds an API to replace existing async jobs. The new job is queued if the old job cannot be replaced (e.g. if the old job is already run).

• Goal of the PR
  See above
• How does the PR work?
  The async engine attempts to replace the existing queue if possible and falls back to queueing the new job.
  Job cancellation is trivially implemented based on this by replacing the old job with an empty function.
• 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
• Implement job cancellation
• Add tests for
  • Replacing jobs before the job is run
  • Replacing jobs after the job is complete
  • 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.