Make GraphImpl serializable to work around capture
Revert changes to examples/.../PageRankUtils.scala

Reverts to 04d83fc37f9eef89c20331c85291a0a169f75e6d:examples/src/main/scala/org/apache/spark/examples/bagel/PageRankUtils.scala.
Unwrap Graph.mapEdges signature
Vid -> VertexID
Pid -> PartitionID