There's no need to create an instance of such class for every call to
one of its methods. The balance is computed each time anyway, so it'll
always be up-to-date.
This keeps the `OrderBalance` abstraction but removes the old code now
that the feature is enabled for all users in all instances and there are
no bugs reported. It's become dead code.
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).
This model concerns helps us put together this related methods. Although
it doesn't provide any encapsulation yet, it makes a bit easier to
consider them all next time we need to change this implementation
somehow. It's a bit of an illusion but it feels like we are making this
God object model a bit smaller.
It also gives more room for documentation that will aid future devs.
