code: long getSequenceSize(SegmentKelp segment)
    {
      int tailPid = _tailPid.get();
      
      long size = 0;
      
      for (int i = 0; i <= tailPid; i++) {
        Page page = _pages.get(i);
        
        if (page != null && page.getSegment() == segment) {
          size += page.size();
        }
      }
      
      return size;
    }
docs: Returns the size in bytes of all pages with the given sequence.
  
  Used by the page garbage-collector to determine which segments to
  collect.