Remove bag initialization in SessionManager

Problem/Motivation

    // @todo When not using the Symfony Session object, the list of bags in the
    //   NativeSessionStorage will remain uninitialized. This will lead to
    //   errors in NativeSessionHandler::loadSession. Remove this after
    //   https://www.drupal.org/node/2229145, when we will be using the Symfony
    //   session object (which registers an attribute bag with the
    //   manager upon instantiation).

#2229145: Register symfony session components in the DIC and inject the session service into the request object is long closed but the @todo remains.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet