[Serve] Allow blue-green deployments of the controller / full cluster.

[JGSweets]

Currently, when running update ..., or even update --mode blue_green ..., the controller doesn't update. Currently, one has to start a new service and swap over. This also changes its static port.

Ideally, there could be a command to conduct a blue-green deployment of the controller / full system which would allow the service to stay live while the new controller and replicas (if they can't be transferred) are not in the READY status.