[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.

graphx/src/main/scala/org/apache/spark/graphx/EdgeRDD.scala

@@ -82,12 +82,17 @@ class EdgeRDD[@specialized ED: ClassTag, VD: ClassTag](
     this
   }
 
-  /** Persists the vertex partitions using `targetStorageLevel`, which defaults to MEMORY_ONLY. */
+  /** Persists the edge partitions using `targetStorageLevel`, which defaults to MEMORY_ONLY. */
   override def cache(): this.type = {
     partitionsRDD.persist(targetStorageLevel)
     this
   }
 
+  /** The number of edges in the RDD. */
+  override def count(): Long = {
+    partitionsRDD.map(_._2.size.toLong).reduce(_ + _)
+  }
+
   private[graphx] def mapEdgePartitions[ED2: ClassTag, VD2: ClassTag](
       f: (PartitionID, EdgePartition[ED, VD]) => EdgePartition[ED2, VD2]): EdgeRDD[ED2, VD2] = {
     this.withPartitionsRDD[ED2, VD2](partitionsRDD.mapPartitions({ iter =>