Completions are basically a oneshot flag, which provide a `wait()` call
that blocks until 'finish()' has been called on the completion.
The nice aspect of completions is that because it is a oneshot the order
does not matter - if 'finish()' is called before 'wait()' then 'wait()'
returns immediately. It makes the logic for waiting until a thread is
done finishing up some work easy to implement.
