Fix two issues with Future key selection inside FutureIterator

Summary:
Ref T11968.

• Rekeying natural lists of futures creates more problems than it solves. For now, don't try to deal with this.
• The scope of the future autokey was incorrect (per subclass, not per process).

Test Plan: Ran various future-oriented flows on new HardpointEngine code.

Maniphest Tasks: T11968

Differential Revision: https://secure.phabricator.com/D21046