Make updateCategoryCounts() have better lag checks

Make updateCategoryCounts() have better lag checks

* Add the lag checks to LinksUpdate. Previously, only
  LinksDeletionUpdate had any such checks.
* Remove the transaction hook usage, since the only two callers are
  already lag/contention aware. Deferring them just makes the wait
  checks pointless and they might end up happening all at once.
* Also set the visibility on some neigboring methods.
* Clean up LinksUpdate $existing variables in passing. Instead of
  overriding the same variable, use a differently named variable
  to avoid mistakes.

Bug: T95501
Change-Id: I43e3af17399417cbf0ab4e5e7d1f2bd518fa7e90