App and Function deploys go live, or fail immediately with a clear error

Production now runs the address router and the controller loop that switch an App or Function to a new version and route its address to its copies. Previously a deploy could stay in switching until the next deploy replaced it, and the App's address returned 404. Where that controller is not running, POST /v1/apps/{id}/deploys, /rollback and :restart (and the same Function routes) now return 501 feature_unavailable immediately and change nothing.