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ARROW-16964: [C++] TSAN error in asof-join-node tests (#13639)

Converted `total_batches_` and `batches_processed_` to atomic variables since they are accessed simultaneously by the input received and process threads.  This was leading to a TSAN failure as described in the JIRA.

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>