[SPARK-1931] Reconstruct routing tables in Graph.partitionBy

905173df introduced a bug in partitionBy where, after repartitioning the edges, it reuses the VertexRDD without updating the routing tables to reflect the new edge layout. Subsequent accesses of the triplets contain nulls for many vertex properties.

This commit adds a test for this bug and fixes it by introducing `VertexRDD#withEdges` and calling it in `partitionBy`.

Author: Ankur Dave <ankurdave@gmail.com>

Closes #885 from ankurdave/SPARK-1931 and squashes the following commits:

3930cdd [Ankur Dave] Note how to set up VertexRDD for efficient joins
9bdbaa4 [Ankur Dave] [SPARK-1931] Reconstruct routing tables in Graph.partitionBy

(cherry picked from commit 56c771cb)
Signed-off-by: Reynold Xin <rxin@apache.org>