cubicweb #3797430 Add the concept of "visits" [open]

Currently the cookie sent to the browser allow to track an authenticated session, or an anonymous one. It does not persist through an authentication process, meaning we cannot easily provide a continuity in the UX between before and after logging in.

This can be an issue with, for example, a basket that is filled as an anonymous user and is payed after he/shee logs in.

It also makes it more difficult to track the users from their landing page to their authenticated session. In the case of A/B testing, and more generally to study the user paths in the application, it is a problem.

The solution to all this is to implement the notion of visit, that most of the web framework already use to keep track of a browser session (out of my mind : TG, pyramid, pylons...)

The session could then use on the visit API and associate a user to the current visit when he/she logs in. It also has the interest of not sending an internal id (the session id) outside of the application.

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