Cache for every Image instead of requiring Image In prepareAssets() mehod

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In Apple's document, " You access the contents of a fetch result with the same methods and conventions used by the NSArray class. Unlike an NSArray object, however, a PHFetchResult object dynamically loads its contents from the Photos library as needed, providing optimal performance even when handling a large number of results" in PHFetchResult. The method to improve performance is require Image in collectionView:CellForItem method