Made LinksUpdate::updateLinksTimestamp() use a more correct timestamp

* This can matter if page Y uses templates A and B, someone edits A,
  someone edits Y, someone edits B, an A-refreshLinks job on Y runs
  (using cache), then a B-refreshLinks job runs on Y. Previously,
  the later job could incorrectly no-op itself due to the high
  page_links_updated value on Y.

Change-Id: I984d2d1fc5aa075f3e3ead4921deaffbdc20e963
Merge "Made LinksUpdate::updateLinksTimestamp() use a more correct timestamp"