private static void countUsingDocIdIteration(byte[] nPostingListsForDocument, List<int[]> nonCachedPostingLists) {
        for (int[] docIds : nonCachedPostingLists) {
            for (int docId : docIds) {
                ++nPostingListsForDocument[docId];
            }
        }
    }
Counts the number of posting lists per document id.
Caches the most expensive posting list in a bit vector.

@author bjorncs
predicate-search/src/main/java/com/yahoo/search/predicate/index/CachedPostingListCounter.java