Authorize.net: IPN could fail if multiple payment processors

This is basically the same thing as one of the pieces CRM-18107 touched on for PayPal: if you have more than one Authorize.net payment processor in your database, IPN will assume it's the first one it finds, and recurring payments through the others will appear to fail.

This is an issue for multi-site setups: the lookup does not check for the domain, so only the first Authorize.net processor across all domains will work.