[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)
sql('DROP FUNCTION IF EXISTS _fsopen(bytea)')
sql('DROP FUNCTION IF EXISTS fspath(bigint, text, text)')