Structure enhancement of the shuffle reader

[jealous]

The code of shuffle reader is not well structured and lack the capability of inserting tests for those classes. The separation of responsibility is not clear. Re-structure the code to meet the following requirements:

• Insert the error handling and dump logic in the correct place and test the dump logic in the unit test.
• Remove the state in SplashShuffleFetchIterator and make it a case class.
• Extract the SplashShuffleFetcher class which is responsible for:
  • Track the resource usage and resource cleanup of the current partition.
  • Error handling and trace information of the current partition.
  • Data transformation related the current partition.
• Extract the iterator wrapper in SplashShuffleReader to separate functions
  • getAggregatedIterator adds the combiner logic into the iterator
  • getSortedIterator adds the sorter logic into the iterator.
• Move all sorter related metrics tracking logic into the sorter itself.