* Refresh the default payment method only if the active one is unavailable
* Add new type for serializable data store payment method
* Update uses of PaymentMethods/ExpressPaymentMethods to the plain version
* Add tests for setAvailablePaymentMethods action
* Update data store key in tests
* Rename payment-method-data-context to payment-event-context and tidy up types and things
* Rename uses of PAYMENT_METHOD_DATA_STORE_KEY to PAYMENT_STORE_KEY
* Rename __internalInitializePaymentMethodDataStore and unsubscribeInitializePaymentMethodDataStore
* Rename unsubscribeInitializePaymentMethodDataStore