BUG-509: simple cleanup of DataTreeModification

Rename rootModification to rootNode to clarify what it is
Use field updated to fix seals to save some memory

Change-Id: I0fe7f58a3c15613c15198830b77bc28a84a9006a
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-509: simple cleanup of DataTreeModification"