Adding migration of existing cookies to new sessions.

If you're using the Riak Memory back-end, you can configure a global ttl for the bucket. Objects in the bucket will automatically expire at (time of creation + ttl), rendering this check unnecessary. It's not exactly a massive performance gain, so I'm not concerned either way.

http://docs.basho.com/riak/latest/ops/advanced/backends/memory/#Configuring-the-Memory-Backend