Ticket 47686 - removing chaining database links trigger valgrind read errors
    
    Bug Description:  Plugins that remove their dse callback from the dse callback
                      function lead to invalid reads in dse_call_callback().
    
    Fix Description:  In dse_call_callback(), save the pointers to the next callback,
                      and its plugin, before we call the callback function.  So in
                      case the callback function removes itself, we are not accessing
                      the freed callback pointer later on.
    
    https://fedorahosted.org/389/ticket/47686
    
    Reviewed by: nhosoi(Thanks!)