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 to Build objects, and it holds a lock while it does it.
Test Plan:
- Created a plan which runs "sleep 2" a bunch of times in a row.
- Stopped, resumed, and restarted it.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran, chad
Maniphest Tasks: T1049
Differential Revision: https://secure.phabricator.com/D7892
