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.
How about:

```
if not cls.buffered_saves:
    return  # nothing to save

cls.flushing = True
```

to avoid set/unset cls.flushing