Make RefreshLinksJob de-duplication more robust

Make RefreshLinksJob de-duplication more robust

* Do not de-duplicate jobs with "masterPos". It either does not
  catch anything or is not correct. Previously, it was the later,
  by making getDuplicationInfo() ignore the position. That made the
  oldest DB position win among "duplicate" jobs, which is unsafe.
* From graphite, deduplication only applies .5-2% of the time for
  "refreshLinks", so there should not be much more duplicated
  effort. Dynamic and Prioritized refreshLinks jobs remain
  de-duplicated on push() and root job de-duplication still applies
  as it did before. Also, getLinksTimestamp() is still checked to
  avoid excess work.
* Document the class constants.

Change-Id: Ie9a10aa58f14fa76917501065dfe65083afb985c