public BucketId toBucketId() {
            return new BucketId(keyToBucketId(key));
        }
For consistent bucket key ordering, we need to ensure that reverse bucket
IDs that have their MSB set actually are compared as being greater than
those that don't. This is yet another issue caused by Java's lack of
unsigned integers.
documentapi/src/main/java/com/yahoo/documentapi/ProgressToken.java