+con #3843 Add ClusterSingletonProxy

(cherry picked from commit ee01a8dffe79b249fb6799e08c8ed677de1b585a)
=sam #3843 Use ClusterSingletonProxy in cluster samples

(cherry picked from commit ad184058777740089ca363e6ed96cfbb3155d5fd)
Merge pull request #2071 from akka/wip-3843-singleton-proxy-backport-patriknw

+con #3843 Add ClusterSingletonProxy (backport)