SPARK-785 [CORE] ClosureCleaner not invoked on most PairRDDFunctions

This looked like perhaps a simple and important one. `combineByKey` looks like it should clean its arguments' closures, and that in turn covers apparently all remaining functions in `PairRDDFunctions` which delegate to it.

Author: Sean Owen <sowen@cloudera.com>

Closes #3690 from srowen/SPARK-785 and squashes the following commits:

8df68fe [Sean Owen] Clean context of most remaining functions in PairRDDFunctions, which ultimately call combineByKey

(cherry picked from commit 2a28bc61009a170af3853c78f7f36970898a6d56)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>