Ticket 49031 - Improve memberof with a cache of ancestors for groups
    
    Bug Description:
        During a group update, memberof computes the ancestors of all impacted
        leafs and groups. To compute the ancestors it lookup recursively and so
        is going to compute the ancestors of a given group as many times there are
        impacted nodes under that group.
    
    Fix Description:
        The ticket implements a cache. It contains for a given group all its
        ancestors, so that if it is needed to recompute the ancestors of that
        group it will take it from the cache.
    
    https://fedorahosted.org/389/ticket/49031
    
    Reviewed by: William Brown (thanks !!!!!)
    
    Platforms tested: F24
    
    Flag Day: no
    
    Doc impact: no