The maximum size of a single message. Note that a lower limit may apply to
messages sent as datagrams, based on the connection MTU and QUIC’s overhead.
Reads a header-prefixed message from a byte slice, and returns the number
of bytes consumed. Returns ProtocolError::ShortBuffer if the buffer
contains a partial message.
Writes a header-prefixed message to a byte slice, and returns the number
of bytes used. Returns ProtocolError::ShortBuffer if the slice doesn’t have
enough capacity.