Getting protocol for session

This simple pach would make possible to know what protocol is in use for a session.

If pulled, onc could later change OpenSslEngine.getSession() with:
SSLSession session = this.session;
        if (session == null) {
            this.session = session = new SSLSession() {
                ...
                public String getProtocol() {
                    SSL.getProtocolForSSL(ssl);
                }

instead of the current code
Please call it getVersion as we usually use the same method names for java and c.