Investigate which classes require registration with Kryo

[tovbinm]

Problem We might have some unregistered classes with Kryo that compromise performance.

Solution Investigate which classes require registration with Kryo by set spark.kryo.registrationRequired=true in TestSparkContext. Then go over failing tests and see if any classes need to be registered in OpKryoRegistrator.

Additional context Only do this once we upgrade to Spark 2.3 - #44