Ticket #50 - server should not call a plugin after the plugin close function is calle
    
    Bug Description:  Its possible that a plugin's pre/post op function can be called as
                      the plugin is shutting down.  It is also possible that a queued plugin
                      task can be run after the plugin is shut down.
    
    Fix description:  Added a new flag to plugin struct to mark when the plugin is closed.
                      This flag is checked when we attempt to call a plugin function.  We also
                      now check if the plugin is closed before issuing a plugin task.  The
                      plugin task change only affected 3 plugins: usn, likedatttrs, & memberOf
    
    https://fedorahosted.org/389/ticket/50