public BucketId getProgress() {
            return progress;
        }
When a bucket has its state kept by the progress token, we need to
discern between active buckets (i.e. those that have been returned by
{@link com.yahoo.documentapi.VisitorIterator#getNext()} but have not
yet been update()'d) and pending buckets (i.e. those that have been
update()'d and may be returned by getNext() at some point)
documentapi/src/main/java/com/yahoo/documentapi/ProgressToken.java