Improve handle semantics with HandlePool / HandleList
Ref T7689, which discusses some of the motivation here. Briefly, these methods are awkward:
This moves us toward better semantics, less awkwardness, and a more reasonable attack on T7688 which won't double-fetch a bunch of data.
- Added unit tests.
- Converted one controller to the new stuff.
- Viewed countdown lists, saw handles render.
Reviewed By: btrahan
Maniphest Tasks: T7689
Differential Revision: https://secure.phabricator.com/D12202