Fix min max bug

This changes the algorithm from the old min/max one to one where it has a window range it fills in, then jumps forward to get a new window.

I'm testing it in my datahub on a real example, and have offered to try it with a customer too.

Would like a code review meanwhile though!

Before merging and deploying:
- [x] Test on customer's example for a day or two in own data hub
- [x] Deploy to customer to check they're happy (in progress at https://scraperwiki.com/dataset/xgitwpm/view/i9tbtd1)
- [x] Code review by someone else
- [x] Test upgrade existing tools in place reliably (in progress at https://scraperwiki.com/dataset/fl6pjqi/view/k8sitgw)
- [ ] When deployed tell users in checklist at https://github.com/scraperwiki/twitter-search-tool/issues/27#issuecomment-42315978