Allow ApplicationSearch to be offset-paged

Summary: Ref T4365. It's not practical to cursor-page all engines; allow main search engines to be offset-paged. Basically, this comes down to setting a flag and then doing a couple of tiny things differently.

Test Plan: Used this two diffs from now.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4365

Differential Revision: https://secure.phabricator.com/D8121