Improve handle semantics with HandlePool / HandleList

Summary:
Ref T7689, which discusses some of the motivation here. Briefly, these methods are awkward:

• Controller->loadHandles()
• Controller->loadViewerHandles()
• Controller->renderHandlesForPHIDs()

This moves us toward better semantics, less awkwardness, and a more reasonable attack on T7688 which won't double-fetch a bunch of data.

Test Plan:

• Added unit tests.
• Converted one controller to the new stuff.
  • Viewed countdown lists, saw handles render.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7689

Differential Revision: https://secure.phabricator.com/D12202