added SessionService

added session service:

- [x] finish the code
- [ ] gather feedback for my changes
- [x] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | no
| Doc           | https://github.com/sulu-cmf/docs/blob/master/developer-documentation/000-foundation/phpcr-session-factory.md