[server] move connection pooler initialization logic to get_cnxset()
This avoid complex logic in Repository initialization.
from cubicweb.predicates import is_instance
from cubicweb.hooks import notification
class FolderUpdateHook(notification.EntityUpdateHook):
__select__ = (notification.EntityUpdateHook.__select__
& is_instance('Folder'))
order = 100 # late trigger so that metadata hooks come before.