gnutls: don't try to reuse a TLS session with a different SNI

Use the Server Name Indication as part of the session ID so that we
don't try to share sessions between different virtual hosts on the
same machine. This causes problems on some servers, which I think is a
server bug, but...

https://bugs.launchpad.net/bugs/823325 and the end of
https://bugzilla.gnome.org/show_bug.cgi?id=581342