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.

Round up cwnd left to the maximum UDP packet size to avoid small
packet.

This commit separates key installation function into 2 and let TLS
stack install rx and tx keys separately for handshake and 1RTT keys.
This change is aligned to the new BoringSSL API and GnuTLS API.  I
expect that OpenSSL will follow this change.  It also removes side
argument from crypto API if it can be inferred by
ngtcp2_conn_is_server.

Fixes #224

This commit adds special handling for the very first Initial packet
which is corrupted and discarded.  If server receives such packet, it
has to drop connection silently without sending any CONNECTION_CLOSE.
Then valid packet might come later.  Keeping connection state is
harmful because corrupted packet has wrong SCID, which drops later
valid packet because mismatched SCID.  New error code
NGTCP2_ERR_DROP_CONN is added which tells server application to drop
connection silently.