[SPARK-13749][SQL][FOLLOW-UP] Faster pivot implementation for many distinct...

[SPARK-13749][SQL][FOLLOW-UP] Faster pivot implementation for many distinct values with two phase aggregation

## What changes were proposed in this pull request?

This is a follow up PR for #11583. It makes 3 lazy vals into just vals and adds unit test coverage.

## How was this patch tested?

Existing unit tests and additional unit tests.

Author: Andrew Ray <ray.andrew@gmail.com>

Closes #12861 from aray/fast-pivot-follow-up.