2.0.0 RC4

Note: This is a pre-release, there might be breaking changes in the final stable version.

• Added Session::unlockAll() that completely unlocks a session, no matter how many times it has been locked (opened).
• SessionMiddleware adds a callback using Response::onDispose() to call Session::unlockAll(). If the session is locked, this maintains the lock until the response is written or discarded.
• Removed a circular reference in Session that prevented session locks from being released automatically.