Summary:
Depends on D7498.
This implements support for a "build step implementation". Build steps have an associated class name (which makes the class in PHP) and a details field, which is serialized JSON (same as PhabricatorRepository).
This also implements a SleepBuildStepImplementation which just pauses the build for a specified period of seconds.
Test Plan:
Inserted a build step with `insert into harbormaster_buildstep (phid, buildPlanPHID, className, details, dateCreated, dateModified) values ('', 'PHID-HMCP-zkh5w6czfbfpk2gxwdeo', 'SleepBuildStepImplementation', '{"seconds":5}', NOW(), NOW());` (adjusting the build plan PHID as appropriate).
Started the daemon and applied the build plan to a buildable, and saw the daemon take a 5 second delay after creating `SleepBuildStepImplementation`.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley
CC: Korvin, epriestley, aran, chad
Maniphest Tasks: T1049
Differential Revision: https://secure.phabricator.com/D7499
