cache_req: allow shallow copy of result
    
    For enumeration purpose, we want to limit the number of returned results
    
    This is a preparation for changes in NSS responder.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3151
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>