fix integration test MapReduceIT

1. Add byte array serde in RowWritable for Writable interface
2. Add ByteArrayAdapter for gson to serialize and deserialize byte array
3. Fix iterator in MapReduceIT.CountReduce
4. Fix index off-by-one issue in testDumpTableToHDFS
Change VitessInputFormat key type

Make VitessInputFormat emit <NullWritable, RowWritable> instead of
<KeyspaceIdWritable, RowWritable>