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: 
Even cooler is to unpack the recordset straight away like this:
```
result, = cur.fetchone()
```
That's literally the same as doing this:
```
results = cur.fetchone()
assert len(results)==1
result = results[0]
del results
```