[connection] eliminate ClientConnection leftovers * in the documentation * in the docstrings * in code notations (e.g. clt_cnx -> cnx) Related to #3837233. From now, there should be just one ClientConnection left in repoapi.py.

missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/*/cubicweb/skeleton/debian/rules.tmpl)