[server] add a db-namespace option in source definition (closes #1631339) "namespace" is preferred to the postgresql term "schema" to try and avoid confusion with cubicweb's schema. cubicweb now depends on logilab-database >= 1.13.0

[serverctl] add db_transaction context manager

[server] refactor cleanup() code used by "cubicweb-ctl delete" Try to make each logical step (i.e. "delete database" and "delete user") a bit more visible and explicit. This is a preparatory work to easily plug the "delete namespace" feature.