Fix bug in BacklinkCache: the lack of an ORDER BY clause in getLinks(), combined...

Fix bug in BacklinkCache: the lack of an ORDER BY clause in getLinks(), combined with the lack of sensible indexes on the categorylinks table, was causing partition() to return starts and ends scaterred randomly across the result set. For large jobs, many partitions end up being large, causing HTMLCacheUpdate::doPartialUpdate() to repartition, thus requeueing jobs in an infinite recursive loop.

The BacklinkCache bug was there since r47317, but was relatively harmless until r54841 introduced the infinite loop issue.