siv: set key directly with gnutls

siv: set key directly with gnutls

A new function is provided by the latest gnutls (should be in 3.7.5) to
set the key of an AEAD cipher. If available, use it to avoid destroying
and creating a new SIV instance with each key change.

This improves the server NTS-NTP performance if using gnutls for SIV.