Leaving/down cluster node should aggressively reject handshakes

[ktoso]

If a node is notified that it is down (or it is leaving by itself), it should aggressively reject any new nodes trying to handshake with it.

(/me happy with how specific each of those words is in our impl 😉)

[ktoso]

Otherwise nodes could start talking to it when many are being downed and joining, causing much more churn in membership than needs to be