Add rolling function as options

[Japjappedulap]

A new option rollingFunction was added. This will determine if the cookie will be set on a request, similar to the rolling option.

Session are rolled by default only on the store, without their .cookie being altered. On each request, a touch is made to the store, meaning that the object's expire is reseted and will live longer than the user's session, leading to different expire dates between the store and the user session. This behaviour has already been reported.

Using rolling will set-cookie with the same session ID, but with reseted maxAge, this is the desired behaviour, as the session is extended and the expires are equal between the client and the server.

Somehow, we need to be able to conditionally or manually roll sessions. This is where rollingFunction comes in handy.