Bring back GnuTLS TLS 1.2 session resumption
I removed support for TLS 1.2 session resumption in 3dbc0817, but TLS 1.2 will be around for a long time to come. !221 (merged) adds a separate TLS session cache for TLS 1.3 vs. 1.2 and it should be very little code for GnuTLS to make use of it.