[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)

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raise NotImplementedError("Cannot migrate such an old version. Use intermediate Cubiweb version (try 3.16.x)")