!clu #3617 API improvements related to CurrentClusterState

* Getter for CurrentClusterState in Cluster extension, updated via
  ClusterReadView
* Remove lazy init of readView. Otherwise the cluster.state will be
  empty on first access, wich is probably surprising
* Subscribe to several cluster event types at once, to ensure *one*
  CurrentClusterEvent followed by change events
* Deprecate publishCurrentClusterState, was a bad idea, use sendCurrentClusterState
  instead
* Possibility to subscribe with InitialStateAsEvents to receive events corresponding
  to CurrentClusterState
* CurrentClusterState not a ClusterDomainEvent, ticket #3614
Merge pull request #1912 from akka/wip-3617-cluster-readView-patriknw

+clu #3617 API improvements related to CurrentClusterState