code: function renamenx(key, newkey, req) {
    var exists = this.keyExists(newkey);
    if(!exists) {
      this.rename(key, newkey, req);
    }
    return exists ? 0 : 1;
  }
docs: Rename a key, only if the new key does not exist.
  
  Expected that the command has validated according the rules for
  RENAME.