3.0.0

This release is compatible with amphp/http-server@^3.

• Renamed Storage, InMemoryStorage, RedisStorage, DefaultIdGenerator, and Driver to SessionStorage, LocalSessionStorage, RedisSessionStorage, Base64UrlSessionIdGenerator, and SessionFactory, respectively
• Renamed methods on Session: open() to lock(), save() to commit()
• Added Session::rollback() to undo local changes and re-read the session data from storage
• Added SessionTrainer for examining session changes unit testing