Add CompositePropertyTableDiffIterator

[mwjames]

A composite view of row inserts/deletes computed by PropertyTableRowDiffer for further processing after update.

refs #1018; removes TODO from SQLStore

array (
  'smw_di_number' => 
  array (
    'insert' => 
    array (
      0 => 
      array (
        's_id' => 3668,
        'p_id' => 61,
        'o_serialized' => '1001',
        'o_sortkey' => '1001',
      ),
    ),
    'delete' => 
    array (
      0 => 
      array (
        's_id' => 3668,
        'p_id' => 61,
        'o_serialized' => '123',
        'o_sortkey' => '123',
      ),
    ),
  ),
  'smw_fpt_mdat' => 
  array (
    'property' => 
    array (
      'key' => '_MDAT',
      'p_id' => 29,
    ),
    'insert' => 
    array (
      0 => 
      array (
        's_id' => 3668,
        'o_serialized' => '1/2015/8/16/9/56/53',
        'o_sortkey' => '2457250.9145023',
      ),
    ),
    'delete' => 
    array (
      0 => 
      array (
        's_id' => 3668,
        'o_serialized' => '1/2015/8/16/9/28/39',
        'o_sortkey' => '2457250.8948958',
      ),
    ),
  ),
)