jobqueue: various cleanups to JobQueueFederated

* The relevant partition information is now stored in
  the fields instead of the general information for all wikis.
* Also improved hash ring caching and made getCoalesceLocationInternal()
  more robust.
* Improved availability of sibling queue methods.

Change-Id: I7ac268bb1db4615e0f735576c506593d5688e17e