[SPARK-2534] Avoid pulling in the entire RDD in various operators

This should go into both master and branch-1.0.

Author: Reynold Xin <rxin@apache.org>

Closes #1450 from rxin/agg-closure and squashes the following commits:

e40f363 [Reynold Xin] Mima check excludes.
9186364 [Reynold Xin] Define the return type more explicitly.
38e348b [Reynold Xin] Fixed the cases in RDD.scala.
ea6b34d [Reynold Xin] Blah
89b9c43 [Reynold Xin] Fix other instances of accidentally pulling in extra stuff in closures.
73b2783 [Reynold Xin] [SPARK-2534] Avoid pulling in the entire RDD in groupByKey.