In order to continue the efforts of #335411, we aim to break the complex issue into more digestible parts.
Step 1: We contribute an implementation of Symfony's Session object to core but don't include any code that uses it.
#1858196: Turn session.inc into a service leveraging symfony session components
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)
Secure Sessions: This issue (as of yet, an unsolved problem)