Add an option to clean up cluster members that are not known to peer discovery backend

[michaelklishin]

Nodes that are not known to peer discovery backends could be periodically purged (removed from the cluster). This feature been available in rabbitmq-autocluster for a while. We decided that it's worth moving to the core
per @Gsantomaggio's suggestion.

This feature will be opt-in, not opt-out, just like in rabbitmq-autocluster.