[MEDIUM] secure the calling conditions of ->set_server_status_{up,down}

[MEDIUM] secure the calling conditions of ->set_server_status_{up,down}

It's not always obvious for the callers of set_server_status_{up,down}
whether the new state really is up or down. Some flags as well as the
effective weight have to be considered. Let's ensure that those functions
perform the necessary check themselves so that if the state transition
cannot be performed, at least everything is updated as required.