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

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