Tag cron database queries to allow modules to alter conditions of updating index table.

I have a requirement where only content 10 years and younger can be searchable even though all content is published (Google must be used to find content 10 years and older). A strange requirement that means I need to be able to remove some records from the index.

Currently there isn't anyway to do this but DBTNG makes it super easy. I've attached a patch that tags the queries to the database that determine which nodes need to updated and removed making it possible for modules to alter the query that retrieves which nodes to update and delete from the search index.