backend: pruner: limit number of concurrent jobs
    
    Default to `os.cpu_count(), from the multiprocessing.Pool defaults.  The
    previous value 50 made the load too high both on production and devel
    instance.
    
    While on it, I'm adding the `prune_workers` config variable to config
    parser.
    
    I renamed the variable from `max_prune_processes` to `prune_workers` as
    that matches the limit better (first the final number of spawned
    processes is much higher as the workers spawn other processes, and
    second it is rather static number, not maximal number of workers).