[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