We should support caching and resuming TLS sessions, to allow faster TLS handshakes. Both standard style and session-ticket-based (assuming support from the TLS library).

On the client side, this can be handled entirely in the backend, without any new API (and this is preferable, since that's the way NSS does it anyway). On the server side we may want to have some explicit GTlsServerConnection API. (Again, that's how NSS does it, although this may be just because it wants to support apps that need to share session caches between multiple server processes, which is not as likely to be an important use case for gio-based TLS servers...)