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GH-45371: [C++] Fix data race in `SimpleRecordBatch::columns` (#45372)

### Rationale for this change

GH-45371

### What changes are included in this PR?

Use `std::atomic_compare_exchange` to initialize `boxed_columns_[i]` so they are correctly written only once. This means that a reference to `boxed_columns_` is safe to read after each element has been initialized.

### Are these changes tested?

Yes, there is a test case `TestRecordBatch.ColumnsThreadSafety` which passes under TSAN.

### Are there any user-facing changes?

No

**This PR contains a "Critical Fix".**

Without this fix, concurrent calls to `SimpleRecordBatch::columns` could lead to an invalid memory access and crash.
* GitHub Issue: #45371

Lead-authored-by: Colin Schultz <colin@chalk.ai>
Co-authored-by: Colin <31864253+colin-r-schultz@users.noreply.github.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Antoine Pitrou <antoine@python.org>