Move cache for preview to Doctrine Cache

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [x] StructureSerializer
- [x] CacheProvider
- [x] remove temp node

fixes: https://github.com/sulu-cmf/sulu/issues/593

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | none
| BC Breaks     | https://github.com/sulu-cmf/sulu/commit/5eb485813c29fab3b6ed1d51aca9cee09f7c71b9
| Doc           | none
I guess there are also more performant ways to perform a deep copy, in case it should be needed.