Cluster singleton actor pattern, see #2895

* ClusterSingletonManager
* ClusterSingletonManagerSpec multi-node test
* Use in cluster router with single master sample
* Extensive logging to be able to understand what is
  going on
* Java api
* Add cluster dependency to contrib
* Add contrib dependency to sample
* Scaladoc
* rst docs in contrib area, ref from cluster docs

(cherry picked from commit 0c38a76c3748ba3acf9f9741fdc79bca61368ae6)