Extract a few Priority-based helpers around MR jobs

Not sure how controversial these changes will be. It may be a little hard to see how I'm going to use these in rename until I create the PR later today. Let me know if you need to see it now.

/cc @etorreborre @markhibberd @raronson 
It is not optimal, but it _shouldn't_ with scare quotes make a difference as long as the AnyVal does it's job. Which isn't that easy to tell without byte-code hoping - but it should be fine.