Speed improvements

I have over 1000 resources in the site map, which generated more than <b>6 seconds</b> after each cleaning cache.

So, i made some speed improvements:
- replaced xPDO::getCollection() on PDO::fetchAll()
- added pre parse of item chunk by the new method makePlaceholders()

No functionality has been changed instead of better Tickets support. Now site map generates in <b>1.5 seconds</b>.