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()

This is unlikely to wraparound in the real world, but fixing it makes the code easier to fuzz.

gnutls: add stub ngtcp2_crypto_set_local_transport_params

Reported by Peter Wu in:
https://github.com/curl/curl/pull/5443#issuecomment-633138665