Update tax adjustments to always be closed

This prevents Spree from trying to automatically recalculate taxes on its own.
We have our own flow for figuring out when taxes should be recalculated and don't want Spree doing its own thing.
This should make things more robust and also lets us clean up some messy code in our pseudo-tax-calculater (spree/calculator/avatax.rb).
Note that there is a new migration here that needs to be run.

Also update the README to list some known issues.