ngtcp2: lack of support for key update makes QUIC connections fail

[bagder]

Trying a HTTP/3 request with ngtcp2 from current git master fails. It results in an assert in ngtcp2 due lack of support for the updated way ngtcp2 wants key updates done.

@tatsuhiro-t explained (in this comment) what needs to be done:

The new secret is derived from the previous secret. I think curl has to store the first secret somewhere in struct.

curl/lib/vquic/ngtcp2.c

Line 163 in 7627a2d

static int quic_set_encryption_secrets(SSL *ssl,

provide rx_secret and tx_secret. If level == NGTCP2_CRYPTO_LEVEL_APP, store them somewhere. I plan to store these secrets in ngtcp2_conn in order to make this process easier. For now, application has to do extra step.

In ngtcp2_crypto_update_key, the remembered rx_secret and tx_secret are passed to ngtcp2_crypto_update_key as current_rx_secret and current_tx_secret. The function writes new secret and traffic keys and IVs in the provided buffers.
The length of secret is same as the current one. Once new secret is obtained, application should keep it for the next generation of secrets. It can discard current secrets.