It's maybe a bug for clear target by using okhttp3

#### 0. the problem:
The function of GlideApp.with(context).clear(target); was invoked when I try to cancel a url-request. It expected that the url-request was canceled but I found the callback function of target which is named onLoadCleared() worked while the url-request was still in the queue of okhttp3 and it still could load the bitmap from internet. I think it seemed to be a bug.

#### 1. my dependencies:
```
implementation 'com.squareup.okio:okio:2.1.0'
implementation 'com.squareup.okhttp3:okhttp:3.11.0'
implementation 'com.jakewharton:disklrucache:2.0.2'
implementation 'com.github.bumptech.glide:glide:4.8.0'
annotationProcessor 'com.github.bumptech.glide:compiler:4.8.0'
implementation 'com.github.bumptech.glide:okhttp3-integration:4.8.0'
```

#### 2. my test code:
```
// DemoAppGlideModule.java
@Override
public void registerComponents(@NonNull Context context, @NonNull Glide glide, @NonNull Registry registry) {
super.registerComponents(context, glide, registry);
OkHttpClient client = new OkHttpClient.Builder()
.connectTimeout(5, TimeUnit.SECONDS)
.readTimeout(10, TimeUnit.SECONDS)
.writeTimeout(10, TimeUnit.SECONDS)
.retryOnConnectionFailure(false)
.build();
registry.replace(GlideUrl.class, InputStream.class, new OkHttpUrlLoader.Factory(client));
}

// MainActivity.java
@Override
protected void onResume() {
super.onResume();
Random random = new Random();
new Thread() {
@Override
public void run() {
super.run();
// pathSet contains some urls
Iterator iterator = pathSet.iterator();
while (iterator.hasNext()) {
final String path = iterator.next();
// DemoGlideTarget extends SimpleTarget, do nothing but just show log of callback
final DemoGlideTarget target = new DemoGlideTarget(500, 500, path, null, false);
UIHANDLER.post(new Runnable() {
@Override
public void run() {
GlideApp.with(MainActivity.this).load(path).skipMemoryCache(true).diskCacheStrategy(DiskCacheStrategy.NONE).into(target);
}
});
UIHANDLER.postDelayed(new Runnable() {
@Override
public void run() {
GlideApp.with(MainActivity.this).clear(target);
}
}, 60 + random.nextInt(500));

try {
Thread.sleep(random.nextInt(100));
} catch (Throwable e) {
}
}
}
}.start();
}
```

#### 3. my analyze:
When use registry.replace(GlideUrl.class, InputStream.class, new OkHttpUrlLoader.Factory(client));, the MultiModelLoader.MultiFetcher.fetchers contains UrlUriLoader's fetcher and HttpUriLoader's fetcher, so it will create two OkHttpStreamFetcher for each of the url-requests, and the UrlUriLoader's fetcher excutes first. When cancel(clear target) occurs, the OkHttpStreamFetcher.cancel() works, and then comes OkHttpStreamFetcher.onFailure("Cancle"). Eventually MultiFetcher.startNextOrFail() will be called and the HttpUriLoader's fetcher will enqueue a new request into okhttp3's queue although the url-request should have been canceled.

Please check for that?

```