Correct the flow of edit authority when sending messages to HarbormasterBuild objects

Summary:
Ref T13072. Currently, Harbormaster builds react to messages by applying a transaction inline (which can race) that has no real effect.

Later, the BuildEngine picks up the mesasge and applies a real effect, but this isn't transactional.

This is backwards, and makes it more difficult to transition to ModularTransaction and EditEngine. The desired workflow is:

• sending a message just writes to the message table (and queues a worker to process the message);
• the BuildEngine processes the message and applies effects in a transactional way.

Force this into at least roughly the right sequence of behaviors. This paves the way for porting to ModularTransaction, which should allow further cleanup.

Test Plan: Paused, resumed, aborted, and restarted a build. Ran BuildWorkers to process the commands, saw builds update appropriately.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13072

Differential Revision: https://secure.phabricator.com/D21687