Removed the unused shuffleId in ShuffleDependency's constructor.
Removed the mapSideCombine option in CoGroupedRDD.
Removed the mapSideCombine option in partitionBy.
Moved shuffle serializer setting from a constructor parameter to a setSerializer method in various RDDs that involve shuffle operations.
Merge pull request #846 from rxin/rdd

Two minor RDD refactoring