[SPARK-18423][STREAMING] ReceiverTracker should close checkpoint dir when...

[SPARK-18423][STREAMING] ReceiverTracker should close checkpoint dir when stopped even if it was not started

## What changes were proposed in this pull request?

Several tests are being failed on Windows due to the failure of removing the checkpoint dir between each tests.

This is caused by not closed file in `ReceiverTracker`. When it is not started, it does not close it even if `stop()` is called.

```
Test org.apache.spark.streaming.JavaAPISuite.testCheckpointMasterRecovery started
Test org.apache.spark.streaming.JavaAPISuite.testCheckpointMasterRecovery failed: java.io.IOException: Failed to delete: C:\projects\spark\target\tmp\1478983663710-0, took 3.828 sec
    at org.apache.spark.util.Utils$.deleteRecursively(Utils.scala:1010)
    at org.apache.spark.util.Utils.deleteRecursively(Utils.scala)
    at org.apache.spark.streaming.JavaAPISuite.testCheckpointMasterRecovery(JavaAPISuite.java:1809)
    ...
```

```
- mapWithState - basic operations with simple API (7 seconds, 640 milliseconds)
Exception encountered when attempting to run a suite with class name: org.apache.spark.streaming.MapWithStateSuite *** ABORTED *** (12 seconds, 688 milliseconds)
  java.io.IOException: Failed to delete: C:\projects\spark\streaming\checkpoint\spark-b8486e2b-6468-4e6f-bb24-88277d2c033c
  ...
```

## How was this patch tested?

Tests in `JavaAPISuite` and `MapWithStateSuite`.

Manually tested via AppVeyor:

**Before**

- `org.apache.spark.streaming.JavaAPISuite`
  Build: https://ci.appveyor.com/project/spark-test/spark/build/71-MapWithStateSuite-1
  Diff: https://github.com/apache/spark/compare/master...spark-test:188c828e682ec45b75d15c3dfc782bcdc8ce024c

- `org.apache.spark.streaming.MapWithStateSuite`
  Build: https://ci.appveyor.com/project/spark-test/spark/build/72-MapWithStateSuite-1
  Diff: https://github.com/apache/spark/compare/master...spark-test:8f6945d0ccde022a23d3848f6b7fe6da1e7c902e

**After**

- `org.apache.spark.streaming.JavaAPISuite`
  Build started: [Streaming] `org.apache.spark.streaming.JavaAPISuite` [![PR-15867](https://ci.appveyor.com/api/projects/status/github/spark-test/spark?branch=3D74F2D5-B0D5-4E1D-874C-685AE694FD37&svg=true)](https://ci.appveyor.com/project/spark-test/spark/branch/3D74F2D5-B0D5-4E1D-874C-685AE694FD37)
  Diff: https://github.com/apache/spark/compare/master...spark-test:3D74F2D5-B0D5-4E1D-874C-685AE694FD37

- `org.apache.spark.streaming.MapWithStateSuite`
  Build started: [Streaming] `org.apache.spark.streaming.MapWithStateSuite` [![PR-15867](https://ci.appveyor.com/api/projects/status/github/spark-test/spark?branch=C8E88B64-49F0-4157-9AFA-FC3ACC442351&svg=true)](https://ci.appveyor.com/project/spark-test/spark/branch/C8E88B64-49F0-4157-9AFA-FC3ACC442351)
  Diff: https://github.com/apache/spark/compare/master...spark-test:C8E88B64-49F0-4157-9AFA-FC3ACC442351

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #15867 from HyukjinKwon/SPARK-18423.