Warm images after image style flush

I made the process more efficient when using queues. File entities are not being loaded anymore, queue items are being created for all files regardless of them passing validation since validation is being run during queue processing as well. This performance improvement is crucial when updating an image style that has thousands of derivatives, now the whole queueing process is near instant instead of taking 10 minutes or more in my case.

@DieterHolvoet: How we detect if an image style is edited or deleted? I think also delete triggers this hook, which is than a potential problem.

I don't know if this will get into 8.x-1.2 release, but for 8.x-1.3 for sure.

@IT-Cru: The hook_image_style_flush is called during ImageStyle::save() and ImageStyle::delete(). The MR 14 will be problematic in the following:
- When the image style config is deleted.
- When the image style config is imported during a deploy process. The MR 14 could trigger a new batch to run during the configuration import.

Set to needs work, because I think we should think of possible problems which could be pop-up.