Task/initializable flushable step elements

| Q                   | A
| ------------------- | ---
| Bug fix?            | N
| New feature?        | Y
| BC breaks?          | Y
| Tests pass?         | Y
| Changelog updated?  | Y
| Fixed tickets       | 
| Doc PR              |


This PR adds a flush() and an initialize method to ItemStep. These methods are very useful to develop clean connectors.