rgmanager: Fix stop/start race
    
    If a thread exits right as rgmanager gets a request
    to start a new thread, there's a chance that the
    the starting thread will incorrectly have a reference
    to the old thread and not retry when it should.
    
    Resolves: rhbz#752486
    
    Signed-off-by: Lon Hohberger <lhh@redhat.com>