Improve flatScheduleTask

[Lukasa]

swift-nio has multiple implementations of flatScheduleTask which are all basically identical:

        let promise: EventLoopPromise<T> = self.makePromise(file: file, line: line)
        let scheduled = self.scheduleTask(deadline: deadline, task)

        scheduled.futureResult.flatMap { $0 }.cascade(to: promise)
        return .init(promise: promise, cancellationTask: { scheduled.cancel() })

Each of these is meaningfully different, e.g. in whether they take a deadline or a delay, or whether they require sendable closures and results.

@glbrntt has pointed out that the line scheduled.futureResult.flatMap { $0 }.cascade(to: promise), which is common to all of them, is potentially suboptimal. A better implementation would be:

scheduled.futureResult.whenComplete { result in
    switch result {
    case .success(let futureResult):
        promise.completeWith(futureResult)
    case .failure(let error):
        promise.fail(error)
    }
}

We should change the implementations, and then confirm that we do actually see an allocation reduction from the change. Assuming we do, we can safely merge it.