Switch to using protocol buffer serialization for the WriteData message

- This commit also fixes an issues with the NodeToNormalizedNodeBuilder where an empty container node was decode with a leaf node within
- MergeData and WriteData are passed in their serialized forms from the TransactionProxy as well

Change-Id: I22eab6059becd427a9f0fae1a9273c8c4e293ee5
Signed-off-by: Moiz Raja <moraja@cisco.com>
Make CompositeModification serializable using protocol buffers

Change-Id: I3e91452b0244c6adec84c000e83d7f993b2a59b7
Signed-off-by: Moiz Raja <moraja@cisco.com>