code: public int getPlacementForestIndex(DocumentURI uri) {
          return getBucketPlacementId(uri, buckets, NUM_BUCKET, forests.length,
              uForests.size());
      }
docs: return the index to the list of updatable forests (all forest - retired -
  RO/DO)
  
  @param uri
  @return index in the list of updatable forests