Remove map-side combining from ShuffleMapTask.

This separation of concerns simplifies the 
ShuffleDependency and ShuffledRDD interfaces.

Map-side combining can be performed in a
mapPartitions() call prior to shuffling the RDD.

I don't anticipate this having much of a 
performance impact: in both approaches, each tuple
is hashed twice: once in the bucket partitioning
and once in the combiner's hashtable.  The same
steps are being performed, but in a different
order and through one extra Iterator.