applyToEitherAsync
abstract override fun <U> applyToEitherAsync(other: CompletionStage<out T>, fn: Function<in T, U>): DelayCompletionStage<U>
abstract override fun <U> applyToEitherAsync(other: CompletionStage<out T>, fn: Function<in T, U>, executor: Executor): DelayCompletionStage<U>