Currently if we call `streamingContext.stop` (e.g. in a `StreamingListener.onBatchCompleted` callback) when a batch is about to complete, a `rejectedException` may get thrown from `checkPointWriter.executor`, since the `eventLoop` will try to process `DoCheckpoint` events even after the `checkPointWriter.executor` was stopped.

Please see [SPARK-14701](https://issues.apache.org/jira/browse/SPARK-14701) for details and stack traces.

## What changes were proposed in this pull request?

Reversed the stopping order of `event loop` and `checkpoint writer`.

## How was this patch tested?

Existing test suits.
(no dedicated test suits were added because the change is simple to reason about)

Author: Liwei Lin <lwlin7@gmail.com>

Closes #12489 from lw-lin/spark-14701.