MP2-422: Solved a bug that - after restoring an ImportJob from disk - either lead to the ImportJob not completing at all or completing too early
MP2-422: VideoThumbnailer: Increased timeout for ffmpeg from 10 to 30 seconds to avoid VideoThumbnailer not processing every video correctly.
MP2-422: VideoThumbnailer: Changed creation of temp file names to avoid orphaned .tmp files. ToDo: move creation of temp file names to FileUtils class
MP2-422: VideoThumbnailer: Limit number of concurrent ffmpeg processes to 1 even if the VideoThumbnailer is called multiple times in parallel by the new ImporterWorker