[FEATURE] Generate thumbnails by batch

[fabarea]

Add limit when generating thumbnails more or less the same way the Core does for salted passwords. Generating thumbnails is very consuming resource wise. Goal is to avoid monopolizing the CPU too long.

[lorenzulrich]

In Neos CMS, they tackled the problem as follows:

• A thumbnail ViewHelper with a new option "async"
• If this is set and a thumbnail doesn't exist yet, instead of generating the thumbnail and returning the URI, an "asynchronous image URI" is returned.
• If the thumbnail exists, the "normal" URI is returned.

Furthermore, most thumbnails won't be visible in the viewport. It would make sense to lazy-load the thumbnails.

[lorenzulrich]

And AFAIK the original task - generating thumbnails by batch - is already implemented.