public long getBucketCursor() {
        return bucketCursor;
    }
Set the internal number of distribution bits, which wil be used for writing
the progress file and calculating correct percent-wise sub-bucket completion.

Note that simply invoking this method on the progress token does not actually
change any of its bucket structures/counts! <i>This is the bucket source's
responsibility</i>, since only it knows how such a change will affect the
progress semantics.

@param distributionBits new distribution bit value
documentapi/src/main/java/com/yahoo/documentapi/ProgressToken.java