Example with declarative management of transactions for Spring Tx or Java EE

Spring Tx and Java EE implements declarative management of transactions via aspects and interceptors respectively.

Spring:

```java
@Transactional
public String getUserName(int userId) {
// [..]
}
```

There should be example here on how to implement an aspect or interceptor that manages transaction on method level, and how to integrate is with rxjava-jdbc, without explicitly invoking methods like `dependOn()`