Fix old regression in HTMLCacheUpdate de-duplication

Fix old regression in HTMLCacheUpdate de-duplication

* The condition timestamp should be the root job timestamp,
  as the comments imply, though that was not the case. Only
  the SET timestamp should be the current timestamp. As it
  was, jobs effectively did not "see" each other since each
  one would be expecting newer than the ones that ran prior.
  The page_touched condition for the UPDATE query was mostly
  useless, since it was always the current timestamp.
* This problem was a regression from f598ca81e3f40166.

Change-Id: I1398366e87c959be1c98e4db959c41309b0ac1b8