Allow repository service lookups to return an ordered list of service refs

Summary:
Ref T13286. To support request retries, allow the service lookup method to return an ordered list of structured service references.

Existing callsites continue to immediately discard all but the first reference and pull a URI out of it.

Test Plan: Ran git pull in a clustered repository with an "up" node and a "down" node, saw 50% serivce failures and 50% clean pulls.

Maniphest Tasks: T13286

Differential Revision: https://secure.phabricator.com/D20775