Bugfix/increment write count

The count of written items was not updated during job execution.
This PR addresses this issue.

It's mergeable only after https://github.com/laboro/platform/pull/532 has been merged.

```
Bug fix: yes
Feature addition: no
Backwards compatibility break: yes
Unit test passes: yes
Behat scenarios passes: ?
Checkstyle issues: no
Documentation PR: 
Fixes the following jira:
 - PIM-1256
```