[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/snapshot_merge.go b/vendor/go.etcd.io/etcd/etcdserver/snapshot_merge.go
new file mode 100644
index 0000000..4177768
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/etcdserver/snapshot_merge.go
@@ -0,0 +1,100 @@
+// Copyright 2015 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 etcdserver
+
+import (
+	"io"
+
+	"go.etcd.io/etcd/etcdserver/api/snap"
+	"go.etcd.io/etcd/mvcc/backend"
+	"go.etcd.io/etcd/raft/raftpb"
+
+	humanize "github.com/dustin/go-humanize"
+	"go.uber.org/zap"
+)
+
+// createMergedSnapshotMessage creates a snapshot message that contains: raft status (term, conf),
+// a snapshot of v2 store inside raft.Snapshot as []byte, a snapshot of v3 KV in the top level message
+// as ReadCloser.
+func (s *EtcdServer) createMergedSnapshotMessage(m raftpb.Message, snapt, snapi uint64, confState raftpb.ConfState) snap.Message {
+	// get a snapshot of v2 store as []byte
+	clone := s.v2store.Clone()
+	d, err := clone.SaveNoCopy()
+	if err != nil {
+		if lg := s.getLogger(); lg != nil {
+			lg.Panic("failed to save v2 store data", zap.Error(err))
+		} else {
+			plog.Panicf("store save should never fail: %v", err)
+		}
+	}
+
+	// commit kv to write metadata(for example: consistent index).
+	s.KV().Commit()
+	dbsnap := s.be.Snapshot()
+	// get a snapshot of v3 KV as readCloser
+	rc := newSnapshotReaderCloser(s.getLogger(), dbsnap)
+
+	// put the []byte snapshot of store into raft snapshot and return the merged snapshot with
+	// KV readCloser snapshot.
+	snapshot := raftpb.Snapshot{
+		Metadata: raftpb.SnapshotMetadata{
+			Index:     snapi,
+			Term:      snapt,
+			ConfState: confState,
+		},
+		Data: d,
+	}
+	m.Snapshot = snapshot
+
+	return *snap.NewMessage(m, rc, dbsnap.Size())
+}
+
+func newSnapshotReaderCloser(lg *zap.Logger, snapshot backend.Snapshot) io.ReadCloser {
+	pr, pw := io.Pipe()
+	go func() {
+		n, err := snapshot.WriteTo(pw)
+		if err == nil {
+			if lg != nil {
+				lg.Info(
+					"sent database snapshot to writer",
+					zap.Int64("bytes", n),
+					zap.String("size", humanize.Bytes(uint64(n))),
+				)
+			} else {
+				plog.Infof("wrote database snapshot out [total bytes: %d]", n)
+			}
+		} else {
+			if lg != nil {
+				lg.Warn(
+					"failed to send database snapshot to writer",
+					zap.String("size", humanize.Bytes(uint64(n))),
+					zap.Error(err),
+				)
+			} else {
+				plog.Warningf("failed to write database snapshot out [written bytes: %d]: %v", n, err)
+			}
+		}
+		pw.CloseWithError(err)
+		err = snapshot.Close()
+		if err != nil {
+			if lg != nil {
+				lg.Panic("failed to close database snapshot", zap.Error(err))
+			} else {
+				plog.Panicf("failed to close database snapshot: %v", err)
+			}
+		}
+	}()
+	return pr
+}