Multiple concurrent requests results in incorrect 200 responses

If you send multiple concurrent requests like:

```
POST transactions andrios -> lucas (amount 5) 
POST transactions andrios -> lucas (amount 10) 
POST transactions andrios -> lucas (amount 15) 
POST transactions andrios -> lucas (amount 20) 
POST transactions andrios -> lucas (amount 25)
```

You'll get multiple `200 OK` responses, but only one of this transactions actualy succeeded.