Add serialversionUUID to cluster/datastore/modification

Serializable classes need to define this field. Also fixes the use of
raw NormalizedNode type.

Change-Id: I674b57d0eea60c1e015cd35eba6c407d29826216
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Add serialversionUUID to cluster/datastore/modification"