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.
I prefer to keep this equal to `None` since I'm using it to indicate we haven't been given any value to put there.