distgit: fair processing of task from multiple sandboxes
    
    Even users coming "late" should be able to process at least some
    importing tasks quickly, and outrun the large queue generated by other
    users.  This is guaranteed by giving the most priority (priority=1) to
    the first task from each sandbox.  Next priority (2) is given to the
    second task from each sandbox, and so on.
    
    This means that no matter how big the queue is, "my first import task"
    (excluding background jobs!) gets always the highest priority — and will
    be processed relatively quickly.
    
    Merges: #2370