* Create a `NowOrLater` helper type
A replacement for `FutureExt::now_or_never` that ensures that the task
is scheduled for waking up later when the inner future is ready.
* Use `NowOrLater` to fix possible delay bug
Previous usage of `now_or_never` meant that the underlying task wasn't
being scheduled to awake when the `Downloads` stream produced a new
item. Using `NowOrLater` instead fixes that issue.
Janito Vaqueiro Ferreira Filho