Ticket #49 - better handling for server shutdown while long running tasks are active
    
    https://fedorahosted.org/389/ticket/49
    
    Bug Description:  tasks like memberOf fix-up do not check for shutdown events.  In my test this causes a shutdown
                      to hang until the task is complete.  I did not get crash in my tests.
    
    Fix Description:  The only plugins that appear to have lengthy tasks are: usn, linkedattrtrs, and memberOf.  Added checks for g_get_shutdown.
    
    Also cleaned up some code & fixed some indentation issues.