public synchronized void registerUsage(List<PostingList> postingLists) {
        for (PostingList postingList : postingLists) {
            frequency.updateValue(postingList.getDocIds(), 0, v -> v + 1);
        }
    }
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