Use SSL_MODE_RELEASE_BUFFERS if available

Sets SSL_MODE_RELEASE_BUFFERS if available, to keep openSSL memory
usage as low as possible.

For more info, see
http://www.imperialviolet.org/2010/06/25/overclocking-ssl.html
https://www.openssl.org/docs/ssl/SSL_CTX_set_mode.html

See also trac #157

Signed-off-by: Cristian Rodriguez <crrodriguez@opensuse.org>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <5381FEFF.1040609@karger.me>
Signed-off-by: Gert Doering <gert@greenie.muc.de>