(bug 56849) Deprecate dangerous edittime-based content update functions

Existing edit conflict logic relies on second-resolution timestamps,
which can fail in many exciting ways.  This patch provides new support
functions which take an explicit revision ID instead of a timestamp.

No functionality is changed.  Hopefully.

TODO:
* Make use of this code by replacing all edittime and starttime logic
with an explicit baseRevId.

Change-Id: I83cdd4adaaa67f81e933654228eccdc9fcdf9009
Merge "(bug 56849) Deprecate dangerous edittime-based content update functions"