SSL: stop accessing SSL_SESSION's fields directly. SSL_SESSION struct is internal part of the OpenSSL library and it's fields should be accessed via API (when exposed), not directly. The unfortunate side-effect of this change is that we're losing reference count that used to be printed at the debug log level, but this seems to be an acceptable trade-off. Almost fixes build with -DOPENSSL_NO_SSL_INTERN. Signed-off-by: Piotr Sikora <piotr@cloudflare.com>