Automatically convert existing sessions

We've upgraded from Play 1.2.5 on one of our products, but due to the new session cookie format, this instantly logs out all the users that upgrade. We wanted this process to be friction-free for our users.

This patch automatically converts existing sessions the first time they connect to our app with v1.3 of Play, from then onwards the new cookie format will be used.

I'm not sure if this is something that you'd want in the mainline, if you do, here's the pull request.