parser: inject the time for {{REVISIONTIMESTAMP}} on pre-save parse

parser: inject the time for {{REVISIONTIMESTAMP}} on pre-save parse

DerivedPageDataUpdater::prepareContent already locks in the revision
timestamp before insertion, so inject that into the parser options
used for any pre-save parse (e.g for edit filters).

This means that a reparse is no longer needed within in the same save
request to get the post-save canonical output. A parse will still be
required if the edit filter output used an edit stash output, since
the revision timestamp is not set at stash time.

Instead of using vary-revision, add a vary-revision-timestamp flag
for the revision timestamp words. The month/day/hour variants retain
their prior optimizations for allowing edit stash output reuse for
the post-save canonical output.

Change-Id: Ic2c13db4d21197c79a89de0de56745ca32918eb6