Formalize usage of default up handler in MuxUpHandler

Make the "default UpHandler" a formal part of the Muxer interface, and then have MessageDispatcher follow some fixed rules as to how it installs an UpHandler (either as the sole UpHandler for a Channel, or as the default UpHandler for a Muxer, or not at all, letting a subclass like MuxMessageDispatcher or MuxRpcDispatcher install add a scoped up handler to the Muxer.