[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)
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/