Etcd support (in-progress)

There's some bug that's making this not work yet, and my commit history is a total mess, but here's a good start for getting etcd tested with jepsen.

@titanous also put together a Vagrantfile that uses docker to spin up 5 containers for the database to make it much easier to actually get jepsen running: https://gist.github.com/titanous/f779900327f19cd8665b

I'm definitely won't have time to advance this for at least a week.