Allow express session to use volos cache as a session store

[theganyo]

See:
https://github.com/apigee/api-platform-samples/blob/master/sample-proxies/oauth-advanced/login-app/apiproxy/resources/node/lib/apigee-session-store.js

And:
https://github.com/apigee/api-platform-samples/blob/master/sample-proxies/oauth-advanced/login-app/apiproxy/resources/node/app.js