ObservationServiceImpl#onNotification incorrect logging details

[AlexITC]

The onNotification method is logging wrong details when the observation is not found for the request.

The current log has the observation instead of the token, and the token instead of the registration:

        // get observation for this request
        Observation observation = registrationStore.getObservation(regid, coapResponse.getToken());
        if (observation == null) {
            LOG.error("Unexpected error: Unable to find observation with token {} for registration {}", observation,
                    coapResponse.getToken());
            return;
}

[sbernard31]

Good catch ! Thx !
Fixed by #424