yang-data-api: NormalizedNodeStreamWriter extensibility

NormalizedNodeStreamWriter API interface has a rather unfortunate design flaw, which limits its usability in case of leaf nodes, leafset entry nodes and anyxml nodes, in particular it does not define a window when such nodes are logically 'open' and extension events (specified by other interfaces) may be called.

Design a new interface which will make sure all nodes need to be terminate with a call to endNode. New extension can then work by defining that things like 'emitAttributes' can be called on an open node, without having two sets of methods.