[repo] straightforward bootstrap sequence. Closes #2841188 * kill fill_schema method which was doing to much different things, in favor of a deserialize_method which is simply reading schema from the db and returning it * set_schema doesn't bother with cubes This allows to easilly follow what's going on and so to cleanup init_cnxset_pool to get the following boostrap order: 1. load cubes (implies loading site_cubicweb) 2. load schema 3. init data sources 4. init cnx sets