uTP Fast Open

[ghazel]

Similar to: https://tools.ietf.org/html/rfc7413

We already have extension bits, so this should be fairly similar to TCP. For the listening side, a new utp_setsockopt() option just like TCP_FASTOPEN can be used during the utp_call_on_accept callback. For the connecting side, utp_setsockupt() option like TCP_FASTOPEN_CONNECT tells the utp_connect() to "succeed" immediately and the fire utp_write() will initiate the connection with the initial data.