[SPARK-2306]:BoundedPriorityQueue is private and not registered with Kry...

Due to the non registration of BoundedPriorityQueue  with kryoserializer, operations which are dependend on BoundedPriorityQueue are giving exceptions.One such instance is using top along with kryo serialization.
Fixed the issue by registering BoundedPriorityQueue with kryoserializer.