[SPARK-18219] Move commit protocol API (internal) from sql/core to core module

## What changes were proposed in this pull request?
This patch moves the new commit protocol API from sql/core to core module, so we can use it in the future in the RDD API.

As part of this patch, I also moved the speficiation of the random uuid for the write path out of the commit protocol, and instead pass in a job id.

## How was this patch tested?
N/A

Author: Reynold Xin <rxin@databricks.com>

Closes #15731 from rxin/SPARK-18219.

(cherry picked from commit 937af592)
Signed-off-by: Reynold Xin <rxin@databricks.com>