### Motivation:
In my previous PR https://github.com/apple/swift-nio/pull/2010, I was able to decrease the allocations for both `scheduleTask` and `execute` by 1 already. Gladly, there are no more allocations left to remove from `execute` now; however, `scheduleTask` still provides a couple of allocations that we can try to get rid of.
### Modifications:
This PR removes two allocations inside `Scheduled` where we were using the passed in `EventLoopPromise` to call the `cancellationTask` once the `EventLoopFuture` of the promise fails. This requires two allocations inside `whenFailure` and inside `_whenComplete`. However, since we are passing the `cancellationTask` to `Scheduled` anyhow and `Scheduled` is also the one that is failing the promise from the `cancel()` method. We can just go ahead and store the `cancellationTask` inside `Scheduled` and call it from the `cancel()` method directly instead of going through the future.
Importantly, here is that the `cancellationTask` is not allowed to retain the `ScheduledTask.task` otherwise we would change the semantics and retain the `ScheduledTask.task` longer than necessary. My previous PR https://github.com/apple/swift-nio/pull/2010, already implemented the work to get rid of the retain from the `cancellationTask` closure. So we are good to go ahead and store the `cancellationTask` inside `Scheduled` now
### Result:
`scheduleTask` requires two fewer allocations
3c3e5fc80b