arrow

ARROW-8299: [C++] Reusable "optional ParallelFor" function for optional use of multithreading

We often see code like
```
    if (use_threads) {
      return ::arrow::internal::ParallelFor(n, Func);
    } else {
      for (size_t i = 0; i < n; ++i) {
        RETURN_NOT_OK(Func(i));
      }
      return Status::OK();
```

This patch adds a helper function to do this.

Closes #6870 from liyafan82/fly_0408_pf

Authored-by: liyafan82 <fan_li_ya@foxmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>