Make waking after a future completes propagates to the next future

Make waking after a future completes propagates to the next future

In our `wakers`, if we first `notify` a future, which is then
`poll`ed complete, and then `notify` the same waker again before a
new future is fetched, that new future will be marked as
non-complete initially and wait for a third `notify`.

The fix is luckily rather trivial, when we `notify` a future, if it
is completed immediately, simply wipe the future state so that we
look at the pending-notify flag when we generate the next future.