Move cache for preview to Doctrine Cache

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [x] StructureSerializer
- [x] CacheProvider
- [x] remove temp node


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | none
| BC Breaks     | https://github.com/sulu-cmf/sulu/commit/5eb485813c29fab3b6ed1d51aca9cee09f7c71b9
| Doc           | none
But I think we could also use it to serialize the structure to JSON, and save that into the cache? Don't really see the advantage of having another own implementation here.