Use new FutureIterator instead of Futures
Summary: Ref T6829. Deprecate the Futures() function.
Test Plan: N/A
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T6829
Differential Revision: https://secure.phabricator.com/D11077