Ticket 48891 ns-slapd crashes during the shutdown after adding attribute with a matching rule
    
    Bug Description:
        This is a followup of a previous fix of the same ticket
        During shutdown, plugin close callback is called AND plugins
        structures are freed (plugin_closeall).
        Free of these structure in plugin_closeall was done to avoid
        noise from valgrind (https://fedorahosted.org/389/ticket/47645).
    
        Later, freeing backends (be_cleanupall) calls the plugins cleanup
        callback.
        The problem is that plugin structure have already been freed and
        cleanup callback can be an invalid pointer.
    
    Fix Description:
        The free of plugins structure is postponed after plugin_closeall
        and be_cleanup with a new function plugin_dependency_freeall.
    
        The fix also fixes a leak in plugin_dependency_startall.
    
    https://fedorahosted.org/389/ticket/48891
    
    Reviewed by: Mark Reynolds and Noriko Hosoi (thanks to you !!)
    
    Platforms tested: F23
    
    Flag Day: no
    
    Doc impact: no