Add build step implementation infrastructure and sleep build step.

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