I had the problem with offline nodes while testing. I think it happened when I shut down a node and tried to apply the proposal again, with only the available nodes but without deleting the proposal first. So the shutdown node was still considered a member of the cluster.  We probably don't need this anymore. If we leave it here it could still save us some trouble in a few rare events. But I guess we better drop it to follow kiss.

(Intendation looked appropriate on my workstation, didn't check it in the pull request)