uncork max message size

[miwarnec]

see paul's kcp PR for possible fix.
-> we use the configured window sizes anyway
-> the limit on max message size is artificial
-> some people do like to send larger messages

[miwarnec]

skywind3000/kcp#291

[PhantomGamers]

FWIW I tried using this fix locally and even with this in place the connection breaks when setting the max message size to above 65,535 bytes
PhantomGamers@4f37cc0

(breaks = client sends handshake to host, host receives handshake and sends one to client, client never receives the handshake back)

[miwarnec]

fixed on master!