[SPARK-9577][SQL] Surface concrete iterator types in various sort classes.

We often return abstract iterator types in various sort-related classes (e.g. UnsafeKVExternalSorter). It is actually better to return a more concrete type, so the callsite uses that type and JIT can inline the iterator calls.

Author: Reynold Xin <rxin@databricks.com>

Closes #7911 from rxin/surface-concrete-type and squashes the following commits:

0422add [Reynold Xin] [SPARK-9577][SQL] Surface concrete iterator types in various sort classes.