Remove Koa core?

[andybee]

As the core of Pharrell is essentially one piece of middleware and #1 is related to removing the unnecessary network-stack-to-feed-Lambda implementation, it may ultimately make more sense for Koa compose to merge the few bits together and map straight in to the Node.js HTTP stack directly via the listen method.

I'm not sure how much "magic" Koa is doing for us behind the scenes that make this too much effort. This will probably be clearer once attempting to remove the Lambda handler library for #1.