Update internal embedded listeners to be non blocking

With ISPN-9714 it will be possible to have a listener perform non blocking operations. Also any listener that doesn't return a CompletionStage is invoked in a different thread. We should convert all of our internal listeners to return a CompletionStage if they can, so that we don't have to incur the cost of switching threads to execute them.

We need to ensure that all embedded listeners return a CompletionStage and never block the current thread.