Use `new FutureIterator` instead of `Futures`

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

Test Plan: N/A

Reviewers: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6829

Differential Revision: https://secure.phabricator.com/D11078