[IO] make the behavior of the completer thread configurable

Available behaviors:
  * none - the completer thread is not started

  * schedule (default) - the completer thread will reap and schedule available
                         completions from worker IoContexts

  * 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.