Message classes (bytes 18-19, read as u16be). The class decides the
header length, see headerLength.
LEGACY: number
20-byte header with an encryption marker, used for the nonce request.
MODERN: number
20-byte header with an encryption marker, used for replies to it.
MODERN_WITH_OFFSET: number
24-byte header with a status code and payload offset.
MODERN_WITH_OFFSET_ALT: number
Also 24 bytes with a status code and payload offset.