Change message header to be fixed width or remove it

I propose that we update the message header to be a fixed width or remove it entirely.

If the header exists to simplify implementations, it should be a fixed width.  This can be achieved by padding the size with zeros.  The proposed commit makes the header exactly 7 characters for any message by representing the size in hexadecimal and padding it.

The JSON message size is already capped, so I can see no other reason for including the header upfront other than implementation convenience?

The rest of the core spec needs to be updated to be consistent with any change.