[HOTFIX] Changed caching mechanism for preview to phpcr temp tree

__Tasks:__

- [ ] test coverage
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation
- [x] PHP Backend - ContentMapper, SessionManager, Structure ... (https://github.com/sulu-cmf/sulu/pull/216)
- [x] Media Selection Content Type (https://github.com/sulu-cmf/SuluMediaBundle/pull/67)


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | https://github.com/sulu-cmf/SuluContentBundle/issues/162
| BC Breaks     | none
| Doc           | TODO
@wachterjohannes please only implement the arrayable interface