[SPARK-2538] [PySpark] Hash based disk spilling aggregation

During aggregation in Python worker, if the memory usage is above spark.executor.memory, it will do disk spilling aggregation. 

It will split the aggregation into multiple stage, in each stage, it will partition the aggregated data by hash and dump them into disks. After all the data are aggregated, it will merge all the stages together (partition by partition).
The overhead of splitting the items into small chunks is MUCH lower than that in ExternalMerger, so this limit is smaller than ExternalMerger (0.5 < 0.9). In the worst cases, it will yield all the items after every `batch` number of items.