SPARK-2519 part 2. Remove pattern matching on Tuple2 in critical section...

...s of CoGroupedRDD and PairRDDFunctions

This also removes an unnecessary tuple creation in cogroup.
save the key and value, i.e.
```scala
this.cogroup(other, partitioner).flatMapValues { pair =>
  val vs = pair._1
  val ws = pair._2
  ... same old code ...
}
```