The size/1 callback was added as a non-optional callback in
42b8a29dbae1d626f32bc16dd81a129caf741138 but wasn't added to the
documentation for the ssl_session_cache_api behavior.

Signed-off-by: Steven Danna <steve@chef.io>

If a handshake message is really big it could happen that the ssl
process would hang due to failing of requesting more data from the
socket. This has been fixed.

Also added option to limit max handshake size. It has a default
value that should be big enough to handle normal usage and small
enough to mitigate DoS attacks.