These classes are only serializable to work around closure capture, so their fields should all be marked `@transient` to avoid wasteful serialization.

This PR supersedes apache/spark#519 and fixes the same bug.

Author: Ankur Dave <ankurdave@gmail.com>

Closes #520 from ankurdave/graphx-transient and squashes the following commits:

6431760 [Ankur Dave] Mark all fields of EdgePartition, Graph, and GraphOps `@transient`