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