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ARROW-10966: [C++] Use FnOnce for ThreadPool's tasks instead of std::function

FnOnce drops dependencies on invocation and is lighter weight than std::function

Closes #9134 from michalursa/master

Authored-by: Michal <michal@ursacomputing.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>