Add "modifiedStart" and "modifiedEnd" constraints to "differential.revision.search"

Summary:
Fixes T13386. See PHI1391. These constraints largely exist already, but are not yet exposed to Conduit.

Also, tweak some keys to support the underlying query.

Test Plan: Ran differential.revision.search queries with the new constraints.

Maniphest Tasks: T13386

Differential Revision: https://secure.phabricator.com/D20730