[migration] More resilient migration script Query the information schema to retrieve existing constraint (primary key and unique) and drop them. Thus don't use anymore DROP CONSTRAINT IF EXISTS (not implemented by e.g. old versions of sqlserver). To do so a missing DISTINCT was necessary to avoid duplicates.

[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.