[SPARK-17638][STREAMING] Stop JVM StreamingContext when the Python process is dead

## What changes were proposed in this pull request?

When the Python process is dead, the JVM StreamingContext is still running. Hence we will see a lot of Py4jException before the JVM process exits. It's better to stop the JVM StreamingContext to avoid those annoying logs.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #15201 from zsxwing/stop-jvm-ssc.