* Add new save_subscription_meta() method to allow us to save customer ID both
in Subscriptions 1.n on the original order and in 2.0 on the subscription/s
created for the order)
* Use the new 'woocommerce_scheduled_subscription_payment_simplify' hook and
deprecate use of the old 'scheduled_subscription_payment_simplify' hook by
moving it to the WC_Addons_Gateway_Simplify_Commerce_Deprecated class
* Use core WC methods on the renewal order passed to scheduled_subscription_payment()
to process the payment rather than having to call special Subscriptions API methods
like WC_Subscriptions_Manager::process_subscription_payments_on_order() and
WC_Subscriptions_Manager::process_subscription_payment_failure_on_order()
* Use the original order's total to determine the amount to charge up-front to
save having to call WC_Subscriptions_Order::get_total_initial_payment()
* Use a description for recurring payments that is consistent with standard orders,
i.e. "{Site Name} - Order {order_id}" instead of a description that does not include
the renewal order's ID. This helps link payments to orders (which can then be traced
to subscription/s).
So that if the renewal is for a failed payment, the the new value is set
on the renewal order's `'_simplify_customer_id'` post meta value and can
be copied back to the original order in `update_failing_payment_method()`.
Brent Shepherd
Claudio Sanches
Nicola Mustone
John Clause
Paul Wilde
Mike Jolley
doublesharp
mgiulio
shivapoudel