1. Put the hack logic into SDWebImageDownloadOperation, each different thumbnail image request will have its callback with desired size of image using different decoding call.
2. Progressive Thumbnail decoding (use together) does not have this behavior, because I think progressive thumbnail is really rare and tolerant, people just use the first thumbnail size image for preview and OK.
3. The manager logic refactor with separate steps, focus on clear to understand the complicated logic and maintainess
4. The thumbnail decode/transformed will callback valid image, but with nil data, this matches the behavior as wiki pages.
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