* taskFuture is the future tracking the result of the callable.
     * newFuture is a future that completes after this and all prior tasks are done.
     * oldFuture is the previous task's newFuture.
     * outputFuture is the future we return to the caller, a nonCancellationPropagating taskFuture.
     *
     * newFuture is guaranteed to only complete once all tasks previously submitted to this instance