Conpherence - add querythead conduit method

Summary: nice title. also adds a description to the create thread method which I forgot to add...  Ref T3166.

Test Plan: queried threads by ids, by phids, and by offset / limit tweakage. Got the right stuff!

Reviewers: epriestley

Reviewed By: epriestley

CC: chad, aran, Korvin

Maniphest Tasks: T3166

Differential Revision: https://secure.phabricator.com/D6096