Change max session id length to 256

[JanTvrdik]

• https://github.com/nette/http/blob/master/src/Http/Session.php#L83
• https://github.com/php/php-src/blob/php-7.1.0RC2/UPGRADING#L100-L102

Session ID is generated from CSPNG directly. As a result, Session ID length
could be any length between 22 and 256. Note: Max size of session ID depends
on save handler you are using.