Automated Recurring Billing is broken by contact merge

Auth.net ARB uses civi's internal contact.id as an external foreign key. When contacts are dedupe merged, ARB is not informed of the new contact id.

The simplest fix would be that authorizeIPN is permissive and can follow a link from the deleted contact to the up-to-date contact. Does this pointer already exist in the schema?

Another fix would be to use a key other than contact id. I believe this id is already redundant, since the initial contribution id is also stored in ARB. We could use a hash of the contribution id as the second key, if we are only using the id as error-checking.