title: PHImagemanager requestImageForAsset memory issue
body: <p>I want to use PHImagemanager to get all photos on the device.
  If I set the targetsize too high the app will crash because of memory warnings. So I tested the request without any use of the returned images and set each image to nil, but still app is crashing. I don't know what I'm doing wrong. Can someone help please?</p>
  
  <pre><code>requestOptions = [[PHImageRequestOptions alloc] init];
  requestOptions.deliveryMode = PHImageRequestOptionsDeliveryModeHighQualityFormat;
  requestOptions.synchronous = false;
  assetsOfPhotos = [PHAsset fetchAssetsWithMediaType: PHAssetMediaTypeImage options: nil];
  PHImageManager *manager = [PHImageManager defaultManager];
  @autoreleasepool {
      for (int i = 0; i &lt;= totalImages - 1; i++) {
          PHAsset *asset = assetsOfPhotos[i];
          [manager requestImageForAsset: asset
                             targetSize: CGSizeMake(640, 480)
                            contentMode: PHImageContentModeDefault
                                options: requestOptions
                          resultHandler: ^void(UIImage *image, NSDictionary *info) {
                              image = nil;
                          }];
      }
  }
  </code></pre>
  
  <p>Setting size to 640x480 crash after about 200 images, 320x240 after about 800 images. As a 640x480 image needs 4 times memory then 320x240 image it seems that the app crashes after the same amount of memory that was allocated. So for me this means that I cannot show more images than 200 imags with 640x480 on the test device, because I cannot free allocated memory.</p>