Correct an issue with epoch timestamps in Conduit

Summary:
Fixes T11375. Some validation code was mishandling raw epoch timestamps.

For numeric values larger than 29999999 (e.g., 2999-12-25, christmas 2999), assume the value is a timestamp.

Test Plan: Used maniphest.search to query for modifiedStart, got a better result set and saw the dateModified constraint in the query.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11375

Differential Revision: https://secure.phabricator.com/D16326