[SPARK-20405][SQL] Dataset.withNewExecutionId should be private

## What changes were proposed in this pull request?
Dataset.withNewExecutionId is only used in Dataset itself and should be private.

## How was this patch tested?
N/A - this is a simple visibility change.

Author: Reynold Xin <rxin@databricks.com>

Closes #17699 from rxin/SPARK-20405.