Killing a job does not actually kill it

[moi90]

If I kill an export job ("Clean All (warning !!!)"), progress_msg is "Killed" (red) at first, but the job continues to run and eventually its state is "Done" (green).

If a running job can not be killed, the interface should not allow it.

This relates to #732, #674 and #149.

It seems that the following is not entirely true:

The processes just die when they try to access their task, which was deleted in DB during the user action "Clean task".