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.
Removing the call to _1 isn't hugely helpful IMO; accessor methods get inlined very fast. However, pattern matching adds a whole bunch of potential throw sites and branches and things like that, and we've noticed problems in the past.