Instead of responding with HTTP 503 if image style is already being generated, wait a little bit longer to allow the other process to complete

If another thread is generating the image, we might no longer need the lock to generate the image. We should probably check if the file exists after a timeout. Also, 5 seconds seems quite long. Changed it to 2 seconds.

Hmm, 2 seconds seemed to short after all. Sorry about that. Checking if the file exists again seems like a good idea though. No need to add another lock in that case.