Give ConpherenceThreadQuery a primary table alias

Summary: Fixes T11113. On the 2nd+ page, we could end up with an ambiguous id WHERE clause because we don't define a primary table alias on this query. Define one.

Test Plan:
Changed SearchEngine to return pages of size 5, searched for my threads, toggled to second page, no exception.

Used DarkConsole to examine that second-page query, saw that it had thread.id explicitly instead of id implicitly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11113

Differential Revision: https://secure.phabricator.com/D16080