[Session] fixed wrong started states

Currently the ```started``` flag will not reached the ```false``` state after a session is closed (happens in ```save()```). Also its wrong that the ```clear()``` method changes the ```started``` and ```closed``` state (happens in ```loadSession()```. This PR will fix the problem and make it possible that the session works correctly after multiple requests on a long running php process.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -