[HttpFoundation] NativeSessionStorage regenerate

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8460
| License       | MIT
| Doc PR        | 

Since session_start is called by the regenerate function, then the 'started' flag of NativeSessionStorage have to be set to true. Otherwise, the variable $_SESSION is initiated and the exception "Failed to start the session: already started by PHP ($_SESSION is set)." is thrown.

This can be reproduced by clearing the session data (cookies) before authenticating with a method that does not require csrf (eg. using the confirmation link of FOSUserBundle).