[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.
@uboness I mean that it is called by the transport client going through the transport service directly (as was the nodes info call before), it is not exposed through clients, so java api users can't call it explicitly (the `Client` doesn't expose such api).