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