Since MediaWiki 1.27(?), MediaWiki sessions have no longer been stored on the disk for longer than the initial log in.
We changed this to the database, however, this isn't the best option. It's slow compared to other options such as Redis and Memcached.
With the move to bellow, we have a considerably more memory to play with. Memcached already has four times as much memory allocated. We should consider putting sessions here or in Redis.
The Wikimedia Foundation uses Redis for sessions as it allowed them to maintain sessions while migrating data centre because Redis can be replicated. While we don't currently have plans to move data centre or run multiple production servers, we should always consider scalability when we build things for the future, to make it easier to scale things should the need ever arise.