[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.

pytest
http://hg.logilab.org/master/logilab/common/archive/default.tar.bz2#egg=logilab-common
http://hg.logilab.org/master/yams/archive/default.tar.bz2#egg=yams