[SPARK-19749][SS] Name socket source with a meaningful name

[uncleGen]

What changes were proposed in this pull request?

Name socket source with a meaningful name

How was this patch tested?

Jenkins

[srowen]

OK, how about other sources? it'd be nice to given them toString methods consistently along with other related classes, if you're bothering to do one.

[uncleGen]

@srowen I think this is the only one souce forgotten to name.

[SparkQA]

Test build #73499 has finished for PR 17082 at commit 68349fa.

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

[zsxwing]

Thanks! LGTM. Merging to master.