Update raft vendoring

[aluzzardi]

We're currently too close to release 1.12 to update core dependencies, but we should do this as soon as 1.12 lands.

See #1221 and etcd-io/etcd#6039 for examples.

[aluzzardi]

@aaronlehmann @abronan

I think it makes sense to do this right after 1.12.1 - sounds good?

[aluzzardi]

We have to review proto changes as well.

Also, test a 1.12 manager against a master manager with updated Raft.