message: return ok if merge is successful
diff: @@ -115,6 +115,7 @@ twisterx::io::Status merge(std::vector<std::string> table_ids, const std::string
  arrow::Result<std::shared_ptr<arrow::Table>> result = arrow::ConcatenateTables(tables);
  if (result.status() == arrow::Status::OK()) {
  put_table(merged_tab, result.ValueOrDie());
  + return twisterx::io::Status::OK();
  } else {
  return twisterx::io::Status((int) result.status().code(), result.status().message());
  }
  