Batch links updates performed by refreshLinks jobs

Batch links updates performed by refreshLinks jobs

This should avoid erratic lag spikes that happen as many links are
added and removed via new pages (sometimes bot generated) and edits
that blank pages as well as their reversions.

In the common cases of a modest number of link changes, the entire
update will still happen in one transaction. In any case, link updates
now use a lock to avoid clobbering each other on the same page.

Bug: T109943
Change-Id: Icd453fcc3d28342065893260ad327eae11870245