Implement async / await instead of Bluebird promises

[jeremydaly]

Node 8.10 runtime is now available in Lambda! https://aws.amazon.com/blogs/compute/node-js-8-10-runtime-now-available-in-aws-lambda/

I'd really like to refactor to use async / await instead of Bluebird so that I can remove ALL dependencies from Lambda API. I think we're early enough into this project to to require the 8.10 runtime in our Lambda functions.

I'm currently using a Promise.each() to serialize middleware, so I'll need to figure out a new approach for that. However, this might create an opportunity to allow middleware to optionally process asynchronously. I'll have to think about this some more.

[jeremydaly]

This has been implemented with v0.5. Turns out serializing the middleware was simple. I also removed a bunch of unnecessary placeholders for storing resolves and rejects for Promises. Since async/await uses promises internally, and there is no way to manually resolve an await, I used native promises to keep support for the next() callback in middleware. It essentially just creates the promise and waits for the callback to trigger a resolve.