Remove conn and user_data from ngtcp2_rand in order to use it before
conn is initialized.

Pass struct sockaddr to ngtcp2 so that it knows how to deal with
address change in more detailed manner (e.g., just compare IP address
without port).

ACK frame largest_ack field is always the largest packet number even
if the ACK that contains largest_ack has been acknowledged by a remote
endpoint.