Give HarbormasterBuildUnitMessage a real Query class

Summary: Ref T13088. Prepares for putting test names in a separate table to release the 255-character limit.

Test Plan: Viewed revisions, buildables, builds, test lists, specific tests.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D20179