Provide a callback for Queries to filter objects from alternate result sets

Summary: Ref T2715. `PhabricatorObjectQuery` can theoretically bypass policies on its side-channel result set. This can't actually happen in practice because all the loading mechanisms are filtered, but provide a general way to implement side channel results safely.

Test Plan: Loaded some pages; see next diff.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2715

Differential Revision: https://secure.phabricator.com/D6514