Implement bin/repository thaw for unfreezing cluster repositories

Summary:
Ref T10751. Add support tooling for manually prying your way out of trouble if disaster strikes.

Refine documentation, try to refer to devices as "devices" more consistently instead of sometimes calling them "nodes".

Test Plan: Promoted and demoted repository devices with bin/repository thaw.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10751

Differential Revision: https://secure.phabricator.com/D15768