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ARROW-18383: [C++] Avoid global variables for thread pools and at-fork handlers (#14704)

Initialization order of module globals is undefined.
    
In a particular case, the IO thread pool would first be instantiated at library load, registering an at-fork handler.
Then, only after, the at-fork handlers would be initialized, losing the handler registered just before.


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