Improve Lambda handling

[andybee]

Currently the Lambda handling is solely reliant on the AWS-supplied package that provides a wrapper around the Node.js HTTP library (running it via a socket) and melding the Lambda event and context callback methods in to it.

Whilst that works and provides a simple way to wrap up any Node.js HTTP/Express/Koa app, it might be nicer longer term to remove the dependency, avoid the internal network-stack-through-a-socket implementation and instead create a Koa-compatiable context object from the Lambda event, use koa-compose to create the middleware stack, and then feed the result back via the newly introduce Node 8.10 Lambda async handling.