Also a related question: if the MessageEvent style callback is used,
does (or can) this defer deserialization until getMessage is called,
or skip it altogether (by never calling getMessage)? If not is there a
way to keep a subscriber active, and a callback called when messages
are received, but to not deserialize them? It seems like, e.g.,
ShapeShifter from topic_tools might do the trick--however it doesn't
appear to be documented, is the API stable?