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).

- [x] always use consistent=true in requests
- [x] implement go-etcd's default retry policy
- [x] ensure fleet can endure loss of leader
- [x] clean up logging