Require StAX2 for all XML operations

Recent flux with StAX implementation necessitating yangtools-2.1.8, it has become clear the JRE XML stack is less than perfect and we can easily be subjected to a different implementation. This not only enlarges our testing matrix, but we need to deal with, sometimes incompatible, implementation quirks.

Transition crititical components to use stax2-api, so that we can settle on woodstox as the reference implementation we package. This should provide additional benefits in terms of performance and capability.