Kudu Connector: use new Scan Token API

Kudu 0.9 will include a new feature called Scan Tokens which will simplify planning on the Java side and creating scanners on the C++ side. The API is designed with Impala in mind, so it has a built in serialization method so that scan tokens can be 'created' on the Java side, and then serialized and rehydrated into a scanner. See the design doc for a high level intro to the API, and the MR input format and Spark RDD for usage examples.