Canceling the SDWebImageOperation will also cancel the disk cache query operation

I noticed that with particularly large images, pulling the disk image from cache and decoding it was taking a considerable amount of time, and in cases where large images were used in UITableViews, these queries were stacking up and causing memory pressure and terminated apps.

I added an NSOperation as a sentinel that the parent (SDWebImageManager) could mark as cancelled, so these stacked queries would not execute.