Fix a slow memory leak in long-lived FutureIterator objects, as used by FuturePool

Summary:
See T13572. FutureIterator does not release futures, so long-lived iterators (like the one that FuturePool may build) can end up leaking memory.

This affects the FuturePool used by the daemon overseer.

See T13572 for more discussion.

Test Plan:

• Ran the simple FutureIterator script from T13572. Before: memory held during iteration, script grows without bound. After: memory released, script uses stable memory.
• Ran the overseer with memory logging and an immediate wakeup from hibernation. Before: saw memory usage grow without bound at a rate of ~300MB/day. After: saw memory usage stable.

Differential Revision: https://secure.phabricator.com/D21466