[SPARK-4917] Add a function to convert into a graph with canonical edges in GraphOps

[maropu]

Convert bi-directional edges into uni-directional ones instead of 'canonicalOrientation' in GraphLoader.edgeListFile.
This function is useful when a graph is loaded as it is and then is transformed into one with canonical edges.
It rewrites the vertex ids of edges so that srcIds are bigger than dstIds, and merges the duplicated edges.

[AmplabJenkins]

Can one of the admins verify this patch?

[rxin]

Jenkins, test this please.

[SparkQA]

Test build #24709 has started for PR 3760 at commit 7f8b580.

• This patch merges cleanly.

[SparkQA]

Test build #24709 has finished for PR 3760 at commit 7f8b580.

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

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/24709/
Test PASSed.

[ankurdave]

Thanks! Merged into master.