Make HarbormasterBuild a better source of truth about restarting/resuming/stopping
Summary: Ref T1049. The logic in the BuildEngine is a little different from the logic on the Build itself. Make these more consistent, and make queued commands more private.
Test Plan: Restarted, stopped, and resumed a build.
Reviewed By: btrahan
Maniphest Tasks: T1049
Differential Revision: https://secure.phabricator.com/D7897