[SPARK-9436] [GRAPHX] Pregel simplification patch

Pregel code contains two consecutive joins:
```
g.vertices.innerJoin(messages)(vprog)
...
g = g.outerJoinVertices(newVerts)
{ (vid, old, newOpt) => newOpt.getOrElse(old) }
```
This can be simplified with one join. ankurdave proposed a patch based on our discussion in the mailing list: https://www.mail-archive.com/devspark.apache.org/msg10316.html

Author: Alexander Ulanov <nashb@yandex.ru>

Closes #7749 from avulanov/SPARK-9436-pregel and squashes the following commits:

8568e06 [Alexander Ulanov] Pregel simplification patch

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

@@ -127,28 +127,25 @@ object Pregel extends Logging {
     var prevG: Graph[VD, ED] = null
     var i = 0
     while (activeMessages > 0 && i < maxIterations) {
-      // Receive the messages. Vertices that didn't get any messages do not appear in newVerts.
-      val newVerts = g.vertices.innerJoin(messages)(vprog).cache()
-      // Update the graph with the new vertices.
+      // Receive the messages and update the vertices.
       prevG = g
-      g = g.outerJoinVertices(newVerts) { (vid, old, newOpt) => newOpt.getOrElse(old) }
-      g.cache()
+      g = g.joinVertices(messages)(vprog).cache()
 
       val oldMessages = messages
-      // Send new messages. Vertices that didn't get any messages don't appear in newVerts, so don't
-      // get to send messages. We must cache messages so it can be materialized on the next line,
-      // allowing us to uncache the previous iteration.
-      messages = g.mapReduceTriplets(sendMsg, mergeMsg, Some((newVerts, activeDirection))).cache()
-      // The call to count() materializes `messages`, `newVerts`, and the vertices of `g`. This
-      // hides oldMessages (depended on by newVerts), newVerts (depended on by messages), and the
-      // vertices of prevG (depended on by newVerts, oldMessages, and the vertices of g).
+      // Send new messages, skipping edges where neither side received a message. We must cache
+      // messages so it can be materialized on the next line, allowing us to uncache the previous
+      // iteration.
+      messages = g.mapReduceTriplets(
+        sendMsg, mergeMsg, Some((oldMessages, activeDirection))).cache()
+      // The call to count() materializes `messages` and the vertices of `g`. This hides oldMessages
+      // (depended on by the vertices of g) and the vertices of prevG (depended on by oldMessages
+      // and the vertices of g).
       activeMessages = messages.count()
 
       logInfo("Pregel finished iteration " + i)
 
       // Unpersist the RDDs hidden by newly-materialized RDDs
       oldMessages.unpersist(blocking = false)
-      newVerts.unpersist(blocking = false)
       prevG.unpersistVertices(blocking = false)
       prevG.edges.unpersist(blocking = false)
       // count the iteration