The cookie size should be minimized?
The cookie value currently is encoded in json:

{"secret": "ZUThghsd6ZT76iizizziziufgjj+"}

why not only store "ZUThghsd6ZT76iizizziziufgjj+" ? Is there any reason to use json? Do we really need to store options in there?

https://github.com/koajs/session/blob/master/index.js#L174
