[VOL-2193] Create mocks for Kafka Client and Etcd

This commit consists of:
1) A kafka client mock that implements the kafka client interface
under voltha-lib-go/pkg/kafka/client.go
2) An embedded Etcd server that runs in-process and represents an
Etcd server.

Change-Id: I52a36132568e08c596bb4136918bebcb654a3b99
diff --git a/vendor/go.etcd.io/etcd/etcdserver/api/v3rpc/header.go b/vendor/go.etcd.io/etcd/etcdserver/api/v3rpc/header.go
new file mode 100644
index 0000000..f23b6a7
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/etcdserver/api/v3rpc/header.go
@@ -0,0 +1,49 @@
+// Copyright 2016 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package v3rpc
+
+import (
+	"go.etcd.io/etcd/etcdserver"
+	pb "go.etcd.io/etcd/etcdserver/etcdserverpb"
+)
+
+type header struct {
+	clusterID int64
+	memberID  int64
+	sg        etcdserver.RaftStatusGetter
+	rev       func() int64
+}
+
+func newHeader(s *etcdserver.EtcdServer) header {
+	return header{
+		clusterID: int64(s.Cluster().ID()),
+		memberID:  int64(s.ID()),
+		sg:        s,
+		rev:       func() int64 { return s.KV().Rev() },
+	}
+}
+
+// fill populates pb.ResponseHeader using etcdserver information
+func (h *header) fill(rh *pb.ResponseHeader) {
+	if rh == nil {
+		plog.Panic("unexpected nil resp.Header")
+	}
+	rh.ClusterId = uint64(h.clusterID)
+	rh.MemberId = uint64(h.memberID)
+	rh.RaftTerm = h.sg.Term()
+	if rh.Revision == 0 {
+		rh.Revision = h.rev()
+	}
+}