[SPARK-14513][CORE] Fix threads left behind after stopping SparkContext

## What changes were proposed in this pull request?

Shutting down `QueuedThreadPool` used by Jetty `Server` to avoid threads leakage after SparkContext is stopped.

Note: If this fix is going to apply to the `branch-1.6`, one more patch on the `NettyRpcEnv` class is needed so that the `NettyRpcEnv._fileServer.shutdown` is called in the `NettyRpcEnv.cleanup` method. This is due to the removal of `_fileServer` field in the `NettyRpcEnv` class in the master branch. Please advice if a second PR is necessary for bring this fix back to `branch-1.6`

## How was this patch tested?

Ran the ./dev/run-tests locally

Author: Terence Yim <terence@cask.co>

Closes #12318 from chtyim/fixes/SPARK-14513-thread-leak.