BUG-509: Rename MutableDataTree to DataTreeModification

This name better reflects the semantics of the class. Also add some
javadoc documentation.

Change-Id: I7292c134bf87af8c0518e2905c7bd2b628c37a25
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-509: Rename MutableDataTree to DataTreeModification"