@aaron asked that we capture the TODO items in RESTBagOStuff.php, so I'm creating a task for each.
@Aklapper good question! So, this is a sub-task of T215533, which is about building the MediaWiki objectcache class that will interface with the new session storage service T206016. The work is to make an implementation of the BagOStuff interface, which is the high-level key-value storage internal API. Although *BagOStuff classes are primarily used for caches, session storage isn't strictly a cache. So... suggestions?