Allow changing key of SIV AEAD ciphers

In gnutls-3.6.14 were added the GNUTLS_CIPHER_AES_*_SIV ciphers. It would be nice to have a function to set the key of an existing cipher instance to avoid having to destroy and create a new instance when the key is changing frequently.

This is useful on Network Time Security (NTS) servers. They don't have any client-specific state and use keys provided by the client in the request. When I compare the server performance with GnuTLS and Nettle, there is a difference of about 12%.