[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.

[bdist_wheel]
universal = 1

[check-manifest]
ignore =
  debian
  debian/*
  cubicweb.spec
  __pkginfo__.py