Load only needed data in smart content and exclude current page

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes (new tests)
| Fixed tickets | https://github.com/sulu-cmf/SuluContentBundle/issues/172 , https://github.com/sulu-cmf/SuluContentBundle/issues/220
| BC Breaks     | yes => https://github.com/sulu-cmf/sulu-standard/pull/371/files#diff-828ad3929c2c6e3ba47c513ec3625631L2
| Doc           | none