[SPARK-4115][GraphX] Add overrided count for edge counting of EdgeRDD.

Accumulate sizes of all the EdgePartitions just like the VertexRDD.

Author: luluorta <luluorta@gmail.com>

Closes #2975 from luluorta/graph-edge-count and squashes the following commits:

86ef0e5 [luluorta] Add overrided count for edge counting of EdgeRDD.

(cherry picked from commit ee29ef3800438501e0ff207feb00a28973fc0769)
Signed-off-by: Reynold Xin <rxin@databricks.com>