Incorrect max handshake message payload size calculation

[Scratch-net]

As stated in spec,

A Noise handshake message is also less than or equal to 65535 bytes.

It includes plain & encrypted public keys as well as authentication tag, provided by AEAD cipher.

So, this code

	if len(payload) > MaxMsgLen {
		panic("noise: message is too long")
	}

is incorrect. MaxMsgLen depends on DHLen & token/pattern