[session] Ensure access to rql varmaker always mark the session as dirty
When one accesses the rql_varmaker, that's usually to use it. The pb was that when
the varmaker was already in page's data (which is stored as session data),
session storage such as redis won't see that the session data is dirty and has
to be stored back at the end of the request.
To fix this, systematically call set_page_data.