Payment tokens was introduced in 2.6 and was updated to use the new CRUD code while some of the CRUD system was still in flux.
While most things were correct, the prop handling for custom fields (like a card's last 4 digits) were directly calling meta functions, instead of delegating to the data store/parent.
This PR moves these props to `extra_data` and follows the same pattern as product types or order items. It also updates some version tags to 3.0.0. Finally, it adds an additional test for saving meta after a create which looks like it was lacking.
To Test:
* Run `phpunit`.
* Go to the "My Account" tab and add a new payment method. You need a payment gateway that supports this, like Simplify.
* Test the add a payment method flow.
* Make a test purchase using the saved payment method.
Also makes sure to properly unset other tokens when a new one is set as default. Finally, we make sure that a user is always set in our payment token tests (which was broken in some WP versions which alerted us to some broken tests).
* Fix up some comments to match other comments in the WC code base
* Add some missing esc_* functions
* Add a gateway filter to get_customer_tokens + a test for it
Claudio Sanches
Justin Shreve
Mike Jolley
Aristeides Stathopoulos