ImageServiceImpl.getOrCreatePreview(): handle possible race gracefully

[php-coder]

It's possible that 2 threads will look for a preview of the same image, won't found it and both start to generate previews. First, we need to test that it won't produce exception (in case of in-memory database) and file corruptions (in case of using filesystem). Second, we should try to handle this case somehow to reduce double work in this case (can we use ConcurrentMap with image id that is currently in progress?).

Tech debt for: 5937d16 (#387)