Recalculate order taxes after promo adjustments applied

Guys have been working on this every time I get a chance. At this point build is all green. I think it fixes partially #1735 as I haven't touched VAT yet. Do you think it makes sense?

The core change here is make the tax adjustment "open" so we can update its amount after promo adjustments are applied. I think we can use the same approach to VAT. I'd just need to another inject `update_adjusments` method in the right place in OrderUpdater. Please let me know what you think. Since it's changes the OrderUpdater api I'd suggest it goes in master only, in case it's right, with a CHANGELOG entry.

cc @schof