feature: Add session.regenerate()

[sititou70]

To prevent session fixation attacks, it would be useful to have an API to regenerate session ID.

session.regenerate() API replaces the current session ID with a new generated one. Return Promise.

await session.regenerate();
await session.commit(); // if `autoCommit` is set to `false`.

This is similar to express-session's Session.regenerate(callback) and PHP's session_regenerate_id.