code: final BlockLeaf copy()
    {
      BlockLeaf block = new BlockLeaf(getId(), _buffer);
      
      block._blobTail = _blobTail;
      block._rowHead = _rowHead;
      block._rowSortHead = _rowSortHead;
      block._keyMinSort = _keyMinSort;
      block._keyMaxSort = _keyMaxSort;
      
      return block;
    }
docs: Copy the block and reuse the buffer for compact() when the old block
  is clean.