Move all cluster locking logic to a separate class

Summary: Ref T10860. This doesn't change anything, it just separates all this stuff out of PhabricatorRepository since I'm planning to add a bit more state to it and it's already pretty big and fairly separable.

Test Plan: Pulled, pushed, browsed Diffusion.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10860

Differential Revision: https://secure.phabricator.com/D15790