Working queue

As per usual, comments are welcome :)

Fixes #380


I'm not too happy with the name. I mean just the English. It isn't a queue that's working, but a queue holding the jobs that are currently running. Can we rename this to CurrentlyRunningQueue, WorkInProgressQueue, or WIPQueue?

Actually... is this a Queue in the CS sense in the first place? Or just a registry of currently running jobs?