NetSSL_OpenSSL: Support session resumption with TLSv1.3

[obiltschnig]

Some changes are required in NetSSL_OpenSSL to support session resumption with TLSv1.3.

Ideally, the code should be updated to use SSL_CTX_sess_set_new_cb().
If that is not feasible, a call to SSL_get1_session() after a successful two-way shutdown may give a resumable session. There should also be a way to check whether a session is resumable, by wrapping SSL_SESSION_is_resumable().

See https://wiki.openssl.org/index.php/TLS1.3#Sessions for a detailed explanation.