faster queries to url_alias

There was recently an issue in the Pathauto queue related to speeding up queries on the url_alias table which I think might be able to help XML Sitemap as well. See http://drupal.org/node/212327

Basically instead of joining on src = concat('node/', nid)

You join on src = CAST(CONCAT('node/', nid) AS CHAR)

This took query times from 70 seconds to .27 seconds (in one test). So, it's pretty helpful. You have to do some trickery to get it to work on postgresql and there is a chance it only works with postgresql8.1+ but that should be pretty common anyway.