Support locating Subscription ID in all forms coming from PayPal

PayPal sends the Subscription ID as "recurring_payment_id" for transaction types = recurring_payment_*, and as "subscr_id" for transaction types = subscr_*

This follows on the everlasting ambiguity PayPal uses to call recurring payments. Transaction examples:
* subscription created, cancelled, payment -> txn_type = subscr_*
* subscription suspended, suspended due to tryout -> txn_type = recurring_payment_*