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.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7897