Detect if a bottle neck is from dml and expose the SQL directly in excimer UI

[brendanheywood]

• Will likely need a core tracker to expose this cleanly
• we just need something simple like $DB->get_last_sql - the data is already there just private
• ideally also get params out as well as sql
• we can't use the DB while we are in an expensive DB call, so need to stash it and pull it out later, and possibly stash the worst N and then show the worse in the profile (or the worst N)