[ENHANCEMENT] change session store folder

__Tasks:__

- [ ] test coverage
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | none
| Fixed tickets |  fixes #359, fixes #259 
| BC Breaks     | none
| Doc           | none
Not sure if referencing like this is a good idea... would prefer something like `%kernel.cache_dir%/../../sessions`, since the folder would then change accordingly, in case we change the location of the cache folder once...