SWIM - Ability to send node leaving for gracefuly shutdown and faster cluster membership state

[zarusz]

Currently when a node leaves (the app is shutdown normally) it will multicast this to other members.

An improvement could be done for a shutting down Node to:

• Select N random members
• Send the leave message to them
• Shutdown gracefully

In such case the other nodes would get this gossip information and later disseminate it to other members in the cluster, thus avoiding the need for multicast or reliance on failure detection to detect an unavailable member.