Better implicit conversion for magnets

By naming the implicit methods `apply` we make it easier for client code
so that there is no need to import the object with the conversion
methods explicitly.
Use cookie name from the manager

There is no need to access the configuration from the session
directives.
Rename directives traits

What is stateful or stateless is not the session but the session
manager, so the name must reflect that, and must not be counter
intuitive.