Add new "Hardpoint" classes to support request parallelization

Summary:
Depends on D21070. Ref T11968. Adds "yield"-aware query classes for parallelizing service calls.

These will replace the similar (but not yield-aware) "Hardpoint" classes introduced previously. This is an API change but most of the old classes still exist and still do the same thing, just with more "yield" statements.

This just adds a bunch of new code with no callers and no API updates.

Test Plan: See future changes.

Maniphest Tasks: T11968

Differential Revision: https://secure.phabricator.com/D21071