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ARROW-12208: [C++] Add the ability to run async tasks without using the CPU thread pool

Added a serial executor which can run async code serially without needing to create threads.  In addition, modifies the Scanner::ToTable and FileSystemDataset::Write to execute serially.  Scanner::Scan is still non-serial but I think this is OK.  The I/O thread pool is still being used however.  To truly remove all instances of std::thread we'd need to change the I/O context to use the serial executor but let's see if this makes R happy enough first.

Closes #9892 from westonpace/feature/arrow-12208

Lead-authored-by: Weston Pace <weston.pace@gmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>