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: 
Note that you if you follow @peterbe's advice below to use parameterized queries (which you should), you can't use it for table or column names. I don't see the value it's providing in this case anyway.