[Feature]: Thumbnail generation to slow

[yvess]

Feature description

When I do a graphql request for an data object with new images >10 and request thumbnail generation, than I get an HTML error response "Network error: 500: Unexpected token P in JSON at position 0" this is a HTML Pimcore Error, because the thumbnail generation takes to long (2-3min and then the error comes in).

When I do the request a few times, all thumbnails are generated and the request passes.

I see that only one process is used for thumbnail generation, I think because of this sequential generation it takes way too long.

For thumbnail generation in the console you can specify how many processes it should use. Something similar would be great for graphql via datahub as well, e.g. in the datahub configuration.

Issue #514 could also be a solution for this problem, but perhaps harder to build