Speeding up the locale module

Another patch that didn't get into 4.7 was a simple (yet important) one that created an index for the locale database. The locale module can suck you dry in terms of DB calls, so this can provide a huge boost for a site using it. The index is added on a BLOB field named 'source' with a length of 30-40 (AFAIK, this number was just picked at random).

-K