Unicast discovery should favour ping response with master over a ping response without master

For unicast zen discovery don't overwrite a ping response for a node if the previous ping response has a set master and the current response hasn't.

Per single main ping request we maintain the received ping response per node. Each node level ping response is mapped into that. If from a previous node level ping request the response has already been set for a node, it will be overwritten. We give higher value to the latest response. This change makes sure that this doesn't happen if the previous response has a set master and the current response hasn't a set master. Otherwise a node will lose the fact that another node has elected itself as master, the result of that would be that there would multiple master nodes in a single cluster.