Reorganize AbstractNormalizedNodeDataOutput

Reorganize AbstractNormalizedNodeDataOutput

This reorganizes AbstractNormalizedNodeDataOutput so that it
implements all NormalizedNodeDataOutput with final methods,
deferring implementation as need to abstract methods.

This way we have precisely one implementation of public methods,
making it easy to audit for correctness. Furthermore it eliminates
writeAugmentationIdentifier() override in NeonSR2 writer.

It also introduces writeQNameInternal(), which skips header checks,
as the call sites have already performed those checks.

Change-Id: Icb4a7e994c04e3d52bd561d99c7743dfdfd88738
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>