void clearAllBuckets() {
        buckets.clear();
        pendingBucketCount = 0;
        activeBucketCount = 0;
    }
@param bucket BucketId to be split into two buckets. Bucket's used-bits
do not need to match the ProgressToken's current distribution bit count,
as it is assumed the client knows what it's doing and will bring the
token into a consistent state eventually.
documentapi/src/main/java/com/yahoo/documentapi/ProgressToken.java