Clustering: Expose PathArgument writer

In our upcoming clustering work we will be serializing
DataTreeCandidate, which is a PathArgument-indexed tree structure.
Expose PathArgument writer and read methods for later use. Also fix
NormalizedNodeOutputStreamWriter not calling flush() on close().

Change-Id: Ided34a8bc1bc77b5ef357346e3d5b057bffc40c1
Signed-off-by: Robert Varga <rovarga@cisco.com>