Use new FutureIterator instead of Futures

Summary: Ref T6829. Deprecate the Futures() function.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: aurelijus, Korvin, epriestley

Maniphest Tasks: T6829

Differential Revision: https://secure.phabricator.com/D11079