Improve Drogon 2.0 API

This is a placeholder ticket for discussions related to public API changes inside the framework.

As we currently already have a user base, and most people would be annoyed by sudden API changes in minor version upgrades, we can use the upcoming release of Drogon 2.0 (whenever that will be) to also improve the public API where it’s currently suboptimal, and introduce breaking changes with a new major version.