payment: make refresh Dao call plugin specific

We need to look at payment methods plugin by plugin (to avoid marking as
deleted payment methods from other plugins).

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
payment: add note on handling default payment methods

Not all gateways support the concept of default payment method.
In that case, Kill Bill is authoritative.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>