feat: make task_writer to use imutable reference

[ZENOTME]

Context

It seems unreasonable to me to have a mutable reference to create a task_writer, how about make the task_id an atomic variable?

Do we prepare make it be a shared-object. Otherwise, maybe RefCell🤔?

Both LGTM, but I guess RefCell can't be Send

/// Return a task writer used to write data into table.
pub async fn task_writer(&mut self) -> Result<TaskWriter> {
    ...
    self.task_id += 1;
    Ok(task_writer)
}

So this PR use atomic usize instead of usize for task_id, make task_writer() to use imutable reference.

[Xuanwo]

Thanks!