[SPARK-9045] Fix Scala 2.11 build break in UnsafeExternalRowSorter

This fixes a compilation break in under Scala 2.11:

```
[error] /home/jenkins/workspace/Spark-Master-Scala211-Compile/sql/catalyst/src/main/java/org/apache/spark/sql/execution/UnsafeExternalRowSorter.java:135: error: <anonymous org.apache.spark.sql.execution.UnsafeExternalRowSorter$1> is not abstract and does not override abstract method <B>minBy(Function1<InternalRow,B>,Ordering<B>) in TraversableOnce
[error]       return new AbstractScalaRowIterator() {
[error]                                             ^
[error]   where B,A are type-variables:
[error]     B extends Object declared in method <B>minBy(Function1<A,B>,Ordering<B>)
[error]     A extends Object declared in interface TraversableOnce
[error] 1 error
```

The workaround for this is to make `AbstractScalaRowIterator` into a concrete class.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #7405 from JoshRosen/SPARK-9045 and squashes the following commits:

cbcbb4c [Josh Rosen] Forgot that we can't use the ??? operator anymore
577ba60 [Josh Rosen] [SPARK-9045] Fix Scala 2.11 build break in UnsafeExternalRowSorter.