Add "live" publisher and storage to Diviner

Summary:
Ref T988. This adds basics for the non-static publishing target:

  - Storage (called "Live", e.g. `DivinerLiveAtom` to distinguish it from shared classes like `DivinerAtom`).
  - Mostly populate the storage.
  - Some minor fixes and improvements.

Test Plan: Generated docs, looked at DB, saw mostly-sensible output.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D5973