[mysql] separate the "columns" lifetime from writer

[MichaelScofield]

I hereby agree to the terms of the CLA available at: https://databend.rs/dev/policies/cla/

Summary

Make the lifetime of the "columns" field in the struct RowWriter an individual one, instead of being equal to struct RowWriter. In doing so, the following pattern of writing multiple result sets (or affected rows) to the client is made possible without Rust complaining about "variable does not live long enough":

let mut writer: QueryResultWriter = ...
for x in responses {
  let columns = find_columns(x.schema());
  writer = write_mysql_resp(columns, x).await;
}
writer.no_more_results().await;