[SPARK-2124] Move aggregation into shuffle implementations

This PR is a sub-task of SPARK-2044 to move the execution of aggregation into shuffle implementations. 

I leave `CoGoupedRDD` and `SubtractedRDD` unchanged because they have their implementations of aggregation. I'm not sure is it suitable to change these two RDDs.

Also I do not move sort related code of `OrderedRDDFunctions` into shuffle, this will be solved in another sub-task.


Shouldn't this say "for map-side combine"?