Try to fix import statements in etcd.go again. Update etcdv3 client.

[Gaboose]

This seems to fix a panic where duplicate enum is attempted to be
registered with protobuf:

goroutine 1 [running]:
github.com/golang/protobuf/proto.RegisterEnum(...)
        /home/gabrielius/go/pkg/mod/github.com/golang/protobuf@v1.3.3/proto/properties.go:458
github.com/coreos/etcd/auth/authpb.init.0()
        /home/gabrielius/go/pkg/mod/github.com/coreos/etcd@v3.3.13+incompatible/auth/authpb/auth.pb.go:103 +0x1b7

If etcd client is imported twice under different names (go.etcd.io/etcd
and github.com/coreos/etcd).

[abronan]

That issue is really annoying and is serious enough to get people in trouble with the broken versioning. So LGTM. We can change this properly to use 3.5.0 when it is out.

[Gaboose]

Issues blocking the 3.5.0 release for easy reference: https://github.com/etcd-io/etcd/milestone/37