etcd3 backend broken by etcd upstream commit

[stapelberg]

Commit etcd-io/etcd@80d1594 moves github.com/coreos/etcd/mvcc/mvccpb to internal/, hence breaking compilation of backends/etcd3/etcd3.go

[chen-anders]

Looks like they plan to revert the change - etcd-io/etcd#9293