Iterator for batch processing

[francois]

An iterator looking like PropelModelPager, allowing processing in batches of n records for very long lists. Something like:

$books= BookQuery::create()
  ->findForBatch();
foreach ($books as $book) { // makes one query every 10 books using on demand hydration
  // do something
}

Note: the query should use where id > $lastId rather than OFFSET in order to scale with O(lg N) rather than O(n).

[francois]

Watch out for wcomposite primary keys:  http://axonflux.com/lessons-from-scaling-scribdcom-how-to-break-a?c=1