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ARROW-10788: [C++] Make S3 recursive tree walks parallel

Use the AWS SDK async APIs to launch child directory reads concurrently as soon
as we get the required information from a parent read.

Also, similarly issue directory tree deletion commands in parallel.

On this machine, listing the entire directory tree at "s3://mf-nwp-models/arome-france/v2/2020-12-02"
goes down from 12 seconds to 2 seconds (a 6x speedup, for 12944 directory entries).

Closes #8818 from pitrou/ARROW-10788-s3-walk-async

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>