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