Add "harbormaster.querybuilds" Conduit API

Summary:
Ref T4809. This one is more straightforward. A couple of tweaks:

  - Remove the WAITING status, since nothing ever sets it and I suspect nothing ever will with the modern way artifacts work (maybe). At a minimum, it's confusing with the new Target status that's also called "WAITING" but means something different.
  - Consolidate 17 copies of these status names into one method.

Test Plan: Ran some queries via Conduit, got reasonable looking results.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4809

Differential Revision: https://secure.phabricator.com/D8795