New processors are not removed from the queue when deleted

[rhadamanthe]

• Start downloading files from a page (with a lot of items).
• Delete all the items.

Items keep on being downloaded, while they should not.

[rhadamanthe]

Instead of deleting one by one, we should adopt a bulk strategy to reduce the number of messages between the background and the front.

[rhadamanthe]

Not sure it is the problem.
It will need more investigations.