R1:ca5400d14bcb

Implement basic Harbormaster daemon and start builds.

Summary: This implements a basic Harbormaster daemon that takes pending builds and builds them (currently just sleeps 15 seconds before moving to passed state). It also implements an interface to apply a build plan to a buildable, so that users can kick off builds for a buildable.

Test Plan: Ran `bin/phd debug PhabricatorHarbormasterBuildDaemon` and used the interface to start some builds by applying a build plan. Observed them move from 'pending'…