[tests/checkintegrity] use the new connection api * serverctl: the db-check command also uses an internal connection object, which makes more sense in the context of `checkintegrity`. * checkintegrity: rename srvcnx to just cnx (a bit because in the near future, there will be one cnx to rule them all)

../../doc/book usr/share/doc/cubicweb-documentation
../../doc/html usr/share/doc/cubicweb-documentation
../../debian/cubicweb-doc usr/share/doc-base/cubicweb-doc