crontabberise drop_old_partitions; bug 1014128; attempt #2

This addresses bug 1014128. Again. (Previous attempt at rebasing was a horrible disaster. :unamused:) 

`?r` @peterbe and/or @rhelmer :revolving_hearts: 
This is why I advocate for using the TransactionExecutor classes, all of this obnoxious DBAPI2 inanity is taken care of for you...