[SPARK-21696][SS] Fix a potential issue that may generate partial snapshot files

## What changes were proposed in this pull request?

Directly writing a snapshot file may generate a partial file. This PR changes it to write to a temp file then rename to the target file.

## How was this patch tested?

Jenkins.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #18928 from zsxwing/SPARK-21696.