[SPARK-9436] [GRAPHX] Pregel simplification patch

[avulanov]

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/[email protected]/msg10316.html

[SparkQA]

Test build #38845 has finished for PR 7749 at commit 8568e06.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[ankurdave]

Thanks! Merged into master.