The reader parses each message set header as it is processed,
storing the header in the reader stack. The header is a struct
that contains common header fields for the various message set
formats, as well as composed structs for v1/v2 specific fields.

The header stores the magic byte, and it's this field that is
consulted before reading any messages in the current message
set. The magic byte determine whether or not the v1 or v2
read routine is called each time the caller wants to read
a message.

Some debug logging was added, which only gets executed when
the debug field is true.

Note: we move this into a new message_reader.go file to separate
the reader logic from the message type in message.go.