Make CloneImagesOrVideos more efficient

[mzur]

The CloneImagesOrVideos job calls ProcessNewVolumeFiles to generate thumbnails etc. for the new files of a volume. This can be problematic if the volume consists of many large videos or tiled images. If all of these images/videos are processed at the same time, worker hosts can be overwhelmed.

But the clone job can be made more efficient, as all the thumbnail etc data already exists. Instead of calling ProcessNewVolumeFiles, we should implement new jobs "CloneImageThumbnails" and "CloneVideoThumbnails" (or similar). Each of these jobs gets one old image/video and the respective new (cloned) image/video. The CloneImageThumbnails job then copies the image thumbnail of the old image to the location of the thumbnail of the new image. Also, if the old image is tiled, it copies all the tile data to the location of the new image. If either the thumbnail or tile data does not exist, the job calls ProcessNewImage. Similarly, CloneVideoThumbnails copies all the video thumbnails and the new video preview sprites. If any of these does not exist, it calls ProcessNewVideo (on the appropriate queue, see ProcessNewVolumeFiles).