Replace go-etcd

This replaces the dependencies on github.com/coreos/go-etcd and github.com/coroes/etcd with a new local package surprisingly called "etcd". It is only intended to implement the functionality that fleet needs (for now).
Right, then we just end up back at the go-etcd we were replacing.

Hopefully we can strike a middleground with some simple helpers for the most common/more trivial cases.

Relatedly: is it your intention that this client end up becoming the mainstream go etcd client, or is this fleet-specific indefinitely?