Bumping minLength for modern browsers

We bumped maxLength to 20,000 for good browsers a while ago. But in reality it made little difference, because minLength was still 1700. So as soon as a batch hit 1700+ it got sent out.

This change makes the min 17,000 for good browsers, to force larger batch sizes.

**IE:** size is 1700-2000
**good browsers:** size is 17000 to 20000

`BatchSender.flush()` forces the data to get sent no matter what the size, so no fear of leaving behind data.