[IO] make the behavior of the completer thread configurable
Available behaviors:
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none - the completer thread is not started
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schedule (default) - the completer thread will reap and schedule available completions from worker IoContexts
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wakeup - the completer thread will wakeup all workers if it observes completions in a worker IoContext. The Fiber produced by the completion will be scheduled when the worker in which's IoContext the cqe lies reaps its completions.