The Session plugin is the base of two related parts of functionality
required for session management in web applications.

The first part, the State, is getting the browser to repeat back a
session key, so that the web application can identify the client and
logically string several requests together into a session.

The second part, the Store, deals with the actual storage of information
about the client. This data is stored so that the it may be revived for
every request made by the same client.

This plugin links the two pieces together.