[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.
Schema of the data
------------------
First take a look at the data schema_ then try to remember that you are browsing
through a heap of data by applying stylesheets to the results of queries. This
site is not a content management system with items placed in folders. It is an
interface to a database which applies a view to retreived data.
.. _schema: schema