New feature idea: new "real-time" priority task which can pause currently running tasks

I know this will probably not be a _high priority_, but here's an idea for a new feature. What do you think?

- Add a new job priority value "real-time" (like in windows i guess).
- When a real-time priority task gets enqueued, look for the lowest priority tasks, then find the newest one
- Pause this job (if possible, otherwise cancel and place back in inactive?) and let the new task take up its place

My use-case is I still want to use kue as my main task distribution system because I love it, but I want to create synchronous requests to a service (previously they were all asynchronous, api request to upload a task, then listen for a "done" on an endpoint). I want to have the user be able to send one request, and get the answer as response within a few seconds (because my process takes that long).

I would be happy to give implementing this a try, but as this is not a necessity for work, I don't know if I'll be able to invest a lot of time. But who knows, maybe one day I'll try and send you a pull request :)

The issue names no files, tests, or entry points. Start by locating Kue’s priority handling and worker scheduling paths, then determine whether pausing, cancelling, and re-queuing are supported; done would require an agreed real-time priority behavior and tests for its scheduling effects.

Written by the indexing model from the issue text.