message: allow threads to run when doing dispatching
diff: @@ -81,6 +81,7 @@ index = [(i, j) for i, j in zip(df._mgr.blknos, df._mgr.blklocs)]"#,
  })
  .collect();
  + py.allow_threads(|| {
  // start dispatcher
  let ncols = schema.len();
  let queries: Vec<String> = nrows.iter().map(|v| format!("{},{}", v, ncols)).collect();
  @@ -90,7 +91,8 @@ index = [(i, j) for i, j in zip(df._mgr.blknos, df._mgr.blklocs)]"#,
  schema.clone(),
  queries,
  );
  - let _dw = dispatcher.run_checked().expect("run dispatcher");
  + dispatcher.run_checked().expect("run dispatcher");
  + });
  // return dataframe
  let df = locals.get_item("df").expect("get df!");
  