We need to update CacheNotifier to return CompletionStage for all its appropriate methods. We also need to update all the internals to use these appropriately.

Not all notification usages may provide support non blocking, but our listener internals should support non blocking listeners for all.

The simplest way internally is to treat all current listeners as "alien" and invoke them in the notification thread pool. If it is sync we would wait for this task to complete. We would also now allow a listener to return a CompletionStage. If this is returned we will use this operate in a non blocking way.