[server/migractions] finish migration to repoapi objects Changeset 241b1232ed7f (Use repoapi instead of dbapi for cwctl shell, upgrade and db-init) only did half of the job. It left the migration handler with both a session (cubicweb.server.session.Session) and a cnx (cubicweb.repoapi.ClientConnection) attribute with different ideas of what Connection they were talking to. With this change, we: - make the caller responsible of disabling security on the Connection if it wants to - turn mih.session into an actual attribute, set on __init__ - same for cnx (the client connection) - drop the "lazy connection" logic, and establish the connection up-front - always go through the connection instead of the session when we need to talk to the db