Add ref lookup for username symbols

Summary:
Ref T13490. I'm attempting to update "arc amend", but it needs to fetch revision authors to raise an "amending a revision you don't own" error.

Support user-symbol resolution.

Along the way, this introduces some infrastructure for abstracting away iteration over a multi-page Conduit result set.

Test Plan:

• Ran "arc inspect ..." for various "user(...)" queries.
• Set page size to 3 and issued a general query, saw the future page it away properly.

Maniphest Tasks: T13490

Differential Revision: https://secure.phabricator.com/D21092