[Epic] Convert co coroutines to async functions

[dtinth]

The Bemuse project was started in 2015, where async function is not standard yet. At that time, generators and promises are becoming the ES2015 standard, so we used the co library to implement our async functions.

Fast-forward to 2019, now we have async function built into Node.js and web browsers. It's about time to convert the remaining usage of co to async functions!