[Discovery] verify connect when sending a rejoin cluster request

When a master receives a cluster state from another node, it compares the local cluster state with the one it got. If the local one has a higher version, it sends a JoinClusterRequest to the other master to tell it step down. Because our network layer is asymmetric, we need to make sure we're connected before sending.
the comment is not properly phrase? maybe mention that the node might got disconnected?