@Override
    public T next() {
      if (!hasNextValue) {
        throw new NoSuchElementException();
      }
      hasNextValue = false;
      return value;
    }
Provides a key/values (note the plural values) interface out of a KeyValueReader,
needed by ReduceRecordSource when reading input from a key/value source.
ql/src/java/org/apache/hadoop/hive/ql/exec/tez/KeyValuesFromKeyValue.java