Replace koa package(s) with lightweight http implementation

[pmuens]

Right now we use koa for our http functionalities.

However we're not using that much koa-related features right now so it might be worthwhile to reconsider the core Node.js http module as a lightweight replacement.

Additionally koa uses a bunch of new ES6 features which makes it hard to transpile the emulator so that it supports older Node.js versions.

Originally brought up by @ac360 (thanks for that 👍).