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

Remove NGTCP2_ALPN_H3 macro which should be defined by HTTP/3 layer.

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.