[CDAP-857] Temporary refactoring in router to lookup /v3 paths

This is temporary refactoring of ``RouterPathLookup`` to support ``v3`` APIs. Temporary because ``v3`` affect and change a large portion (though not all) of the APIs, to reflect namespace, and I currently do not have a complete list of exactly how each API will be changed to "namespace" it. During the course of the next few weeks, I will update the ``RouterPathLookup.getV3RoutingService()`` method to capture all required changes during namespace implementation. Once done, we could do a complete refactor of this class in a much cleaner way.

Also, this PR has a lot of files primarily because I changed the name of ``Constants.Gateway.GATEWAY_VERSION`` to ``Constants.Gateway.API_VERSION_2`` for consistency.

Testing done:
- [x] All tests in ``RoutePathTest`` pass.
- [x] Tested some APIs in standalone mode
- [x] Played around with the ``SparkPageRank`` app on the UI with these changes and it seemed to work fine.
- [ ] Cluster testing