Cleanup and performance tweaks for BacklinkCache.

* Batch the queries to get the partitions.
* Improved caching of getNumLinks() with $max.
* Added a TTL to the ProcessCacheLRU use for sanity.
* Some small logic cleanups in partitionResult().
* Also cleaned up some code duplication.

Bug: 43452
Change-Id: I0b9710fe222b3d2cb4dc9ab2eeb0758873a8066c
Merge "Cleanup and performance tweaks for BacklinkCache."