Fastify Integration

[SlinkyPotato]

Problem

Allow start up of autohook with Fastify as an alternative option.

Solution

Introduce a new function startFastifyServer() under index.js that follows similar to startServer() but uses Fastify instead of http.createServer.

Result

Faster processing of requests from twitter DMs.

[CLAassistant]

All committers have signed the CLA.