provide a callback to be notified on secret generation
As mentioned in: #826 (comment 213450784)
To install packet protection keys from TLS, we need a way to be notified when a new secret is set in TLS. This can be implemented by providing a callback.
The API would looks something like:
typedef void (*gnutls_secret_hook_func)(gnutls_session_t, unsigned epoch, unsigned incoming, const gnutls_datum_t *secret); void gnutls_session_set_secret_hook_function(gnutls_session_t session, gnutls_secret_hook_func func);