BUG-2967: Expose before-state from DataObjectModification

BUG-2967: Expose before-state from DataObjectModification

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.

Rather than forcing everyone to duplicate data, expose the before-state,
as best available. Note that applications still need to deal with the
fact that the before-state in delete can be null -- at least by issuing
a warning to the operator.

Change-Id: I72c886f384254a6808159eaee612b07083073492
Signed-off-by: Robert Varga <rovarga@cisco.com>