IO requests issued from fibers will be queued into the io_uring of
the worker's IoContext.
Before retrieving the next fiber to run in the DispatchLoop the
worker tries to reap all completions which occurred in its io_uring and
schedules the now runnable fibers.

To process IO completions on sleeping worker threads there exists
a global completer thread.
All worker io_urings register an eventfd which signals ready completions
and insert a read from this eventfd into a global io_uring.
The global completer thread waits for completions on the global io_uring,
which signal available completions on a worker's IoContext and tries to
reap the worker's completions and schedules all now runnable Fibers
on the AnywhereQueue.

The assumption of this design is that it reduces contention on the IO
subsystem and schedules fibers blocked on IO back into the workers
work-stealing queue and not into the global AnywhereQueue.