[SPARK-18751][CORE] Fix deadlock when SparkContext.stop is called in Utils.tryOrStopSparkContext

## What changes were proposed in this pull request?

When `SparkContext.stop` is called in `Utils.tryOrStopSparkContext` (the following three places), it will cause deadlock because the `stop` method needs to wait for the thread running `stop` to exit.

- ContextCleaner.keepCleaning
- LiveListenerBus.listenerThread.run
- TaskSchedulerImpl.start

This PR adds `SparkContext.stopInNewThread` and uses it to eliminate the potential deadlock. I also removed my changes in #15775 since they are not necessary now.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #16178 from zsxwing/fix-stop-deadlock.

(cherry picked from commit 26432df9)
Signed-off-by: Shixiong Zhu <shixiong@databricks.com>