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ARROW-11590: [C++] Move CSV background generator to IO thread pool

The `AsyncThreadedTableReader` was previously creating a dedicated size 1 thread pool for readahead (similar to the way the serial reader behaves).  This allowed for parallel readahead because the futures couldn't get worked on faster than 1 at a time anyways.  However, this introduced a thread outside of both the CPU context and the I/O context.

This PR adds a new AsyncGenerator utility (MakeSerialReadaheadGenerator) which still provides readahead but, unlike MakeReadaheadGenerator, it does not pull in an async reentrant fashion.  This allows the I/O thread pool to be used because the serial logic will take care of ensuring that no more than one request is ever active.

Aside: In implementing this feature we encountered an issue with futures.  The logic in SerialReadaheadGenerator is relying on callbacks being run by Future in a reliable order.  Some changes were made to Future to ensure this.

Closes #9533 from westonpace/feature/arrow-11590

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