R1:1786093c6e41

Replace "Cancel Build" with "Stop", "Resume" and "Restart"

Summary:
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…