Feature/421 43 refund to store credit

[rsisco]

• Add code to create two new AvaTax DB tables
• Add code to populate tables on queue processing
• Remove code that is now unused
• Add code to migrate data from old columns to new tables
• Refactor code to load data from new tables into ext attributes
• Add code to remove unused AvaTax columns from core tables
• Refactor code so new install will create new tables instead of adding columns to core tables

[mttjohnson]

We should review the use of extension attributes for Magento\Sales\Api\Data\CreditmemoInterface in etc/extension_attributes.xml

I'm wondering how this works when used with a SearchResult on a list of multiple CreditMemo results. I'm not sure we have an existing use case for this, but it is something to be aware of at least.

There is some support for extension attributes that utilize a reference table (reference_table, reference_field, join_on_field) http://devdocs.magento.com/guides/v2.0/extension-dev-guide/attributes.html.

[mttjohnson]

You will probably also want to resolve branch conflicts before your final test.