Discovery: master fault detection fall back to cluster state thread upon error

With #7834, we simplified ZenDiscovery by making it use the current cluster state for all it's decision. This had the side effect a node may start it's Master FD before the master  has fully processed that cluster state update that adds that node (or elects the master master). This is due to the fact that master FD is started when a node receives a cluster state from the master but the master it self may still be publishing to other node.

 This commit makes sure that a master FD ping is only failed once we know that there is no current cluster state update in progress.