Remove raw SQL transactions from SitemapIndex

The Sitemap index used raw SQL queries for finding the max id
which has been replaced with ORM search method.

SitemapSectionSQL, which was due for deprecation has been removed

Cherry Picked from f4a592621f4d11923a22f3948a39b8cd52ff5843
Update sitemap to use ActiveRecord pattern

The sitemap generators were not updated to use Active Records. This
patch updates the examples as well on how to use the SiteMap generators
with Active Record.