[repository] move cnxset pool handling to a helper class The class has responsability to handle connections pool operations in a single public attribute 'cnxsets'. On Repository _get_cnxset() and _free_cnxset() are replaced by cnxsets.get() and cnxsets.release(). Drop multiple access to private attributes and methods from outside of Repository.

cubicweb source: source-is-missing cubicweb/web/data/jquery.flot.js line length is * characters (>512)