Fix #995: Parallelize Incremental Optimizer

This could be written with the tail-recursive `||` idiom:
```scala
@tailrec
private final def append[T](acc: AtomicAcc[T], x: T): Boolean = {
  val oldV = acc.get
  val newV = x :: oldV
  acc.compareAndSet(oldV, newV) || append(acc, x)
}
```