Allow users to query feed by a date range

Summary: Ref T12762.

Test Plan:

• Ran queries with start date, end date, both, neither.
• Used EXPLAIN to try to make sure doing the bitshift isn't going to be a performance issue.

{F4978842}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12762

Differential Revision: https://secure.phabricator.com/D18029