ISPN-4867 Single event marshaller support more clearly defined in API

* Based on Dan's feedback, Hot Rod server API made it look as if you could pass a per-factory marshaller but that's not really the case. A single event marshaller is currently supported.
Seems a bit obscure to me, but i'd agree with something like `match (marshaller) { case None => defaultMarshaller ; case _ => marshaller }`