SSL: stop accessing SSL_SESSION's fields directly.

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>