Enable query activediff timestamp
Summary: people want to get the info about how long the revision has been active since the last time the authoer ran 'arc diff'.
Test Plan: call it from conduit console
Reviewers: vrana, epriestley
Reviewed By: epriestley
CC: Girish, aran, epriestley
Differential Revision: https://secure.phabricator.com/D2885