Custom sessionId

I recently updated Yoke and found custom sessionId support was removed. It's required by [mod-auth-mgr](https://github.com/vert-x/mod-auth-mgr) I'm using with Yoke so adding it back. Also making session store protected for possible extended usage.

Paulo, thank you a lot for the hours spent of this framework, please consider adding more extensibility. Currently the code has almost all variables and methods private leading to dirty hacks and full classes copy-pasting instead of neat extension. By now Yoke is the best web framework for vertx, thanks again for your work.