Fix for using MySQL cluster memcached as session backend and supporting fail-over within the cluster.

I was getting very odd "key not found" errors when failing over in a cluster during an active session. The first solution was to remove encryption but that created a new problem of the session storage not being able to rebuild the session data. This update allows for base64 encryption of the pickle object into memcached without an encryption key, allowing the session to be sticky throughout the cluster. I'm open to another solution that allows for encrypting the session.