In #335411: Switch to Symfony2-based session handling we tried to create a patch that re-implement Drupal session handling all in one go. Our efforts in that patch were frequently hindered by a fast moving head. Instead we are trying to create a series of patches that will help land a Symfony 2 based implementation of Sessions while not having to reroll the patch as often.
This patch does just one thing, Register an implementation of the Symfony 2 Session object onto the DI Container. The actual implementation is not complete and will be fleshed out in #1858198: Implement Symfony's Session handling in core and #1858200: Implement secure session with Symfony's Session object. When we finish these three issues, we'll have a working implementation of Symfony 2's session handling in Drupal.
In order to continue the efforts of #335411: Switch to Symfony2-based session handling, we aim to break the complex issue into more digestible parts.
Step 1: This issue
We contribute an implementation of Symfony's Session object to core but don't include any code that uses it.
Step 2: We implement the basics of Session management using Symfony's Session Object.
#1858198: Implement Symfony's Session handling in core
Step 3: We implement complex topics
Lazy Loading (should be automatically provided by Symfony's Session but extra work may be required)
#1858200: Implement secure session with Symfony's Session object