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' to 'building' to 'passed'.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7498