[SPARK-30566][BUILD] Iterator doesn't refer outer identifier named "iterator" properly in Scala 2.13

[sarutak]

What changes were proposed in this pull request?

Renamed an identifier iterator to iter to avoid compile error with Scala 2.13.

Why are the changes needed?

As of Scala 2.13, scala.collection.Iterator has "iterator" method so if an inner class of Iterator means to refer an outer identifier named "iterator", it does not work as we think.
I listed source files that can be affected by that change by find . -name "*.scala" -exec grep -El "new .*Iterator\[.* +{" {} \;
As far as I confirmed util.Utils` is affected.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Existing tests.

[SparkQA]

Test build #116981 has finished for PR 27275 at commit df1af67.

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

[srowen]

Looks fine. I don't remember that one in my local build for 2.13 but who knows :)
I am sitting on a number of changes for 2.13, and happy to work with you on those if you are too.

[HyukjinKwon]

Merged to master.

[sarutak]

@srowen Yeah, I'll tackle those issues for Scala 2.13.