Optimize upgrade_minmaxgrade course detection query to use keys

Environment: RHEL 6
Database: 5.5.32-MariaDB

Testing the fix for MDL-48618 today on a copy of a large client site I identified a problematic query in the upgrade_minmaxgrade function that had been running for several hours creating a rather large tmp file. After consulting with Martin Langhoff we agreed that query joins should be written reverse of what they are currently. This will utilize the existing mdl_gradgrad_ite_ix key and greatly reduce the query time.

After this fix the query went from never completing to completing in under 1 minute.