async.go
func Async(check Check, interval time.Duration) Check {
	return AsyncWithContext(context.Background(), check, interval)
}
func Async(check Check, interval time.Duration) Check {
	return AsyncWithContext(context.Background(), check, interval)
}
// Async converts a Check into an asynchronous check that runs in a background
// goroutine at a fixed interval. The check is called at a fixed rate, not with
// a fixed delay between invocations. If your check takes longer than the
// interval to execute, the next execution will happen immediately.
//
// Note: if you need to clean up the background goroutine, use AsyncWithContext().