I believe this brings us back to stable. A lot of complexity was
learned about juggling priorities.
We were essentially promoting more chunks to urgent than really
needed to be urgent.
So this commit adds a lot more logic to juggle neighbor priorities
and demote their priority once they meet the requirements needed of
them.
This greatly improves the performance of "urgent" chunks".
Fixes#3410Fixes#3426Fixes#3425Fixes#3416
