Add DataObjectModification.getDataBefore()

Not exposing the before-state requires users to track this state themselves. The problem is most visible in the delete case, if the user needs some attributes to reconstruct keys to other structures in order to perform cleanup – they would effectively have to retain a InstanceIdentifier->data map to efficiently perform deletes.

This ability already exists in DataChangeEvents, which DataObjectModification is aimed to replace. Rather than forcing everyone to duplicate data, expose the before-state, as best available.