Ref T1049. Currently you can cancel a build, but now that we're tracking a lot more state we can stop, resume, and restart builds.
When the user issues a command against a build, I'm writing it into an auxiliary queue (HarbormasterBuildCommand) and then reading them out in the worker. This is mostly to avoid race messes where we try to save() the object in multiple places: basically, the BuildEngine is the only thing that writes to Build objects, and it holds a lock while it does it.