Fix two issues with Future key selection inside FutureIterator
- 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