RESPONDER: Make clear {reset_,}idle_timer() are related to client
    
    Those public functions dealing with idle_timer are related to the
    client's timeout. Let's make it explicit as we will introduce more
    timeout handlers (this time, related to the responder itself).
    
    Related:
    https://fedorahosted.org/sssd/ticket/3245
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>