Don't reset cstat->last_tx_pkt_ts which makes assert(earliest_pktns)
fail.

Fully initialize ngtcp2_transport_params

Node.js makes the full `ngtcp2_transport_params` struct available
for storage as a byte array, including any possible padding
in the struct.

Fully initializing the struct with `memset()` makes this deterministic.

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.

Fix potential wraparound in conn_update_recv_rate()