frontend: texlive DoS for DistGit again
    
    Before, even a single long running non-background DistGit import task
    (e.g. for the large SRPMs like texlive) caused that all the background
    import tasks were not processed for a long time.
    
    Newly we _always_ let the dist-git importer(s) process a part of the
    background queue (if any).
    
    Note this is not 100% ideal (several long-time running background jobs
    can delay processing of non-background jobs for some time), so we should
    eventually migrate the "priority queue" logic (WorkerManager
    abstraction) which is already on the Backend side.  Fortunately most of
    the import tasks are processed instantly.
    
    Merges: #2184
    Relates: #2154