Buffer rows via scraperwiki.sql.save

This introduces buffering of `scraperwiki.sql.save` so that we don't defer to dumptruck for every row in a tight loop.

This should have a significant performance impact for premium, where very many users are using the anti-pattern:

```python
for row in data:
    scraperwiki.sql.save([], row)
```

We've measured a performance improvement in excess of 10x and a factor 2000 less write throughput.
This should be an empty list, since we're resetting back to the default state.
Not sure whether dumptruck accepts None as a unique_keys argument.
(Is there a test for this behaviour? I've not looked.)