backend: equivalent priority for concurrent sandboxes
    
    With the recent @copr/PyPI tasks (tens of thousands builds) we observed
    that other builds in @copr/ namespace aren't processed.  That's because
    the WorkerManager.run(timeout=) isn't able to go through that huge bunch
    of tasks in the priority queue -- and because the other (submitted
    later) had lower priority.
    
    Resolves: #2079
    Merges: #2080