DNS-backed cluster

Per discussion on the finaglers mailing list: https://groups.google.com/d/msg/finaglers/HqfNWJF3qZk/mWD-By0MtOgJ

DnsCluster continuously resolves a host name into a set of SocketAddress instances, loosely based on ZK cluster implementation . 

LingeringCluster is a generic Cluster filter that propagates Rem events after a predefined delay, this helps dealing with  firewall that does its own DNS resolution.