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.