FIX for "error setting TTL index on collection: sessions"

What I finally located in [stackexchange thread #22698661](http://stackoverflow.com/questions/22698661/mongodb-error-setting-ttl-index-on-collection-sessions) is:

> essentially Express is receiving connections before the session store is fully connected. The solution is to wait for the connection to occur before allowing your application to start listening.
> You can avoid this problem by using a callback on MongoStore creation, or passing in an already active connection.

So I've turned this little piece inside out, where we attach the mongo store to the express app.