R1:adfe84ffce4f

Add HarbormasterRunnerWorker, for running CI tests

Summary:
This is very preliminary and doesn't actually do anything useful. In theory, it uses Drydock to check out a working copy and run tests. In practice, it's not actually capable of running any of our tests (because of complicated interdependency stuff), but does check out a working copy and //try// to run tests there.

Adds various sorts of utility methods to various things as well.

Test Plan: Ran `reparse.php --harbormaster --trace <commit>`…