Minor Description
VertexRDDImpl uses an RDD[ShippableVertexPartition[VD]] however, GraphXUtils.registerKryoClasses does not register ShippableVertexPartition. This means when running with spark.kryo.registrationRequired set to true, we get a "Class is not registered" exception. This is an issue as it prevents other unregistered classes from being discovered using spark.kryo.registrationRequired as the first unregistered class found halts the whole job. It also potentially decreases the serialised size of the RDD when using Kryo.
