void reset(T value) {
      this.value = value;
      hasNextValue = true;
    }
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