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).

This commit introduces ngtcp2_stream_data_flags.  The signature of
ngtcp2_recv_stream_data is reworked.  More specifically, flags
parameter is added.  The fin parameter is removed and it is replaced
with NGTCP2_STREAM_DATA_FLAG_FIN flag.  NGTCP2_STREAM_DATA_FLAG_0RTT
flag is added to indicate that the data might be replayed.