Resolve all futures inside FutureIterator

Summary:
Depends on D21032. Ref T11968. Currently, "Future" and "FutureIterator" can both resolve futures.

Treat "Future->resolve()" as sugar on resolving an iterator of size 1.

Test Plan: Ran tests, created this revision.

Maniphest Tasks: T11968

Differential Revision: https://secure.phabricator.com/D21033