koa 3.0 roadmap
There are lots of PRs are blocked due to breaking change, and maybe it's time to make a plan for koa 3.0 now, if anyone has other suggestion, please post in this issue. :)

- https://github.com/koajs/koa/issues/1044
- https://github.com/koajs/koa/issues/1008
- https://github.com/koajs/koa/issues/998
- https://github.com/koajs/koa/issues/904
- https://github.com/koajs/koa/pull/1011
