Fix srcache fetch status in logs

Replace: `upstream_cache_status` with `srcache_fetch_status`

```nginx
    log_format rt_cache '$remote_addr $upstream_response_time $srcache_fetch_status [$time_local] '
    '$http_host "$request" $status $body_bytes_sent '
    '"$http_referer" "$http_user_agent"'
    ' $request_time' ;
```
Check `upstream_response_time` as well.