[SPARK-21491][GRAPHX] Enhance GraphX performance: breakOut instead of .toMap

## What changes were proposed in this pull request?

`Traversable.toMap` changed to 'collections.breakOut', that eliminates intermediate tuple collection creation, see [Stack Overflow article](https://stackoverflow.com/questions/1715681/scala-2-8-breakout).

## How was this patch tested?
Unit tests run.
No performance tests performed yet.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: iurii.ant <sereneant@gmail.com>

Closes #18693 from SereneAnt/performance_toMap-breakOut.