Add owners.query method

Summary:
Allow callers to find all packages/paths owned by a given
user/project.

Test Plan:
Used conduit api page with --
user owner: PHID-USER-6ce1c976b86e5f3c34f6 (tried both loadPackagesFromProjects
true/false)

proj owner: PHID-PROJ-r5wnmmaawqsn4tvjmqm4

repo/path: E, /tfb/trunk/www/flib/privacy. Checked both path.getowners
and owners.query

(all of these are defined internally at fb)

Reviewers: epriestley, btrahan, nh

Reviewed By: btrahan

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2482