[SPARK-2455] Mark (Shippable)VertexPartition serializable

VertexPartition and ShippableVertexPartition are contained in RDDs but are not marked Serializable, leading to NotSerializableExceptions when using Java serialization.

The fix is simply to mark them as Serializable. This PR does that and adds a test for serializing them using Java and Kryo serialization.

Author: Ankur Dave <ankurdave@gmail.com>

Closes #1376 from ankurdave/SPARK-2455 and squashes the following commits:

ed4a51b [Ankur Dave] Make (Shippable)VertexPartition serializable
1fd42c5 [Ankur Dave] Add failing tests for Java serialization
[SPARK-2441][SQL] Add more efficient distinct operator.

Author: Michael Armbrust <michael@databricks.com>

Closes #1366 from marmbrus/partialDistinct and squashes the following commits:

12a31ab [Michael Armbrust] Add more efficient distinct operator.
[SPARK-2405][SQL] Reusue same byte buffers when creating new instance of InMemoryRelation

Reuse byte buffers when creating unique attributes for multiple instances of an InMemoryRelation in a single query plan.

Author: Michael Armbrust <michael@databricks.com>

Closes #1332 from marmbrus/doubleCache and squashes the following commits:

4a19609 [Michael Armbrust] Clean up concurrency story by calculating buffersn the constructor.
b39c931 [Michael Armbrust] Allocations are kind of a side effect.
f67eff7 [Michael Armbrust] Reusue same byte buffers when creating new instance of InMemoryRelation