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 fixes the assertion failure which happens when
acknowledgement of earlier packet increases CRYPTO offset.
Previously, the offset is always the first CRYPTO frame offset in the
queue (pktns->crypto.tx.frq).  But passing this into
ngtcp2_pkt_crypto_max_datalen() underestimates the space because
acknowledgement might increase offset; it increases the required space
to serialize offset.

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.

Deal with initial_scid, retry_scid, and original_dcid.  We need to set
and get QUIC transport parameters timely manner.  Therefore, transport
parameter operations are now done by ngtcp2_crypto library.
Applications do not need to do set it manually.  Added several
callback function implementations to ngtcp2_crypto library which can
be directly passed to ngtcp2_callbacks: client_initial, recv_retry,
and recv_client_initial.  Installation of initial packet number space
encryption keys is now handled by ngtcp2_crypto library.  They are
done in ngtcp2_crypto_client_initial_cb, ngtcp2_crypto_recv_retry_cb
and ngtcp2_crypto_recv_client_initial_cb.