Eliminate per-row based dummy iterator creation

HIVE-15580 introduced a dummy iterator per input row which can be eliminated. This is because SparkReduceRecordHandler is able to handle single key value pairs. We can refactor this part of code 1. to remove the need for a iterator and 2. to optimize the code path for per (key, value) based (instead of (key, value iterator)) processing. It would be also great if we can measure the performance after the optimizations and compare to performance prior to HIVE-15580.