This will let us branch by abstraction. All existing calls to
`#outstanding_balance` will go through `OrderBalance` hence, will
check the feature toggle.
Note that by default, `OrderBalance` will end up calling
`#old_outstanding_balance`. As the name states, that's exactly what
`#outstanding_balance` was so far. This means no consumers will see any
change in behavior. We just added on item in the call stack (sort of).
The original payment may not be valid because its credit card may be
expired. Stripe gives this as a valid scenario returning a success and
we should do too.
When creating the credit payment we end up validating all sources in
a chain as follows.
```
Payment being persisted -> source payment -> original credit card.
```
The source payment was valid when created (It would not be persisted
otherwise) but its source card may now be expired, and that's legit.
There was also an issue with the `#invalidate_old_payments` callback. It
was causing the original payment to be validated again and thus the
credit payment failed to be persisted due to the original credit card
being expired. Switching this callback to use `#update_column` skips
validations and so we don't validate the source payment. We only care
about the state there, so it should be fine.
