[CLIENT] Add internal liveness action

This commit adds a very lightweight action to the transport
serivce that allows to fetch clustername and the discovery node
from a node. This is used by transport clients to test liveness of
a node without using the nodesinfo API which can be blocking if management
threadpools are busy.
Oh, I see. In the old code, there was the option that the node info call will not return any nodes. OK. I still like the exception better, but  that's a question of taste. I'm fine with leaving as is.