Goal: We want to be able to establish that when there is a ServerDescription that is not unknown, that it has an rtt.

When an isMaster comes in after the initial handshake, there is no currentRtt by design, and there is no guarantee that there is a lastRtt if the ServerDescription is not Unknown. We would like to ensure ReplicaSetMonitorManager::validateHost is called after dropConnections before any new connections are established.