public BucketProgress getNext() {
            throw new IllegalStateException("getNext() called on ExplicitBucketSource");
        }
Provides an explicit set of bucket IDs to iterate over. Will immediately
set these as pending in the {@link ProgressToken}, as it is presumed this set is
rather small. Changing the distribution bit count for this source is
effectively a no-op, as explicit bucket IDs should not be implicitly
changed.
documentapi/src/main/java/com/yahoo/documentapi/VisitorIterator.java