R1:1a59cae743c2

Update some Phabricator behaviors for changes to Futures

Summary:
Depends on D21053. Ref T11968. Three things have changed:

- Overseers can no longer use FutureIterator to continue execution of an arbitrary list of futures from any state. Use FuturePool instead.
- Same with repository daemons.
- Probably (?) fix an API change in the Harbormaster exec future.

Test Plan:
- Ran "bin/phd debug task" and "bin/phd debug pull", no longer saw Future-management related errors.
- The Harbormaster future…