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: 
actually, since this cronapp is created using the ```with_single_postgres_tranasction```  (renamed to ```as_single_postgres_transaction```)  transaction management is handled for you.  An app with that decorator should not use either commit or rollback.   Exit with ```return``` for commit, exit with an exception for rollback.