[feat] Replace npm dependency "only"
## Describe the feature

The package "only" has no license information in the npm package. So it lets audits for licensing fail.

A ticket was created, but no reaction. For me the the package seems to be unmaintained.
https://github.com/tj/node-only/issues/17

I suggest to add this trivial function to koa or to replace it with another npm package.

## Checklist

- [x] I have searched through GitHub issues for similar issues.
- [x] I have completely read through the README and documentation.
