[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.

Author: ankit.bhardwaj <ankit.bhardwaj@guavus.com>

Closes #1299 from AnkitBhardwaj12/BoundedPriorityQueueWithKryoIssue and squashes the following commits:

a4ae8ed [ankit.bhardwaj] [SPARK-2306]:BoundedPriorityQueue is private and not registered with Kryo