Make it easier to construct multi-column paging clauses from Query classes

Summary:
We currently have two giant messes for paging across multiple columns (usually because one column is not unique), and I'm about to add a third for Maniphest.

Provide a more structured way to build these A > a OR (A = a AND B > b) clauses.

Test Plan: Set page size to 2 for Differential and Diffusion and paged forward and backward with a bunch of different orders set. Pages worked as expected.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2625

Differential Revision: https://secure.phabricator.com/D6971