DYNDNS: improve nsupdate_msg_add_fwd()
    
    Update nsupdate_msg_add_fwd() to group commands by address family
    processed IP address belongs to.
    
    It's better to group removing old A addresses and adding new A
    addresses in a single transaction. Same goes for AAAA addresses.
    
    Separate transaction for A and AAAA addresses updates are important
    because server might block updates for one of these families and thus
    the update even for the non-blocked address family would unnecessarily
    fail.
    
    For more details please see:
    https://fedorahosted.org/sssd/wiki/DesignDocs/DDNSMessagesUpdate
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2495
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>