+clu #3617 API improvements related to CurrentClusterState

* Getter for CurrentClusterState in Cluster extension, updated via
  ClusterReadView
* 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 materialized events corresponding to CurrentClusterState

I kept the last thing as a separate commit so far, in case you don't like it.