Add SASL handshake negotiation

This patch adds the mandatory SASL handshake for SASL negotiation.

broker.go

@@ -514,6 +514,53 @@ func (b *Broker) responseReceiver() {
 	close(b.done)
 }
 
+func (b *Broker) sendAndReceiveSASLPlainHandshake() error {
+	rb := &SaslHandshakeRequest{"PLAIN"}
+	req := &request{correlationID: b.correlationID, clientID: b.conf.ClientID, body: rb}
+	buf, err := encode(req)
+	if err != nil {
+		return err
+	}
+
+	err = b.conn.SetWriteDeadline(time.Now().Add(b.conf.Net.WriteTimeout))
+	if err != nil {
+		return err
+	}
+
+	bytes, err := b.conn.Write(buf)
+	b.updateOutgoingCommunicationMetrics(bytes)
+	if err != nil {
+		Logger.Printf("Failed to send SASL handshake %s: %s\n", b.addr, err.Error())
+		return err
+	}
+	b.correlationID++
+	//wait for the response
+	header := make([]byte, 8) // response header
+	n, err := io.ReadFull(b.conn, header)
+	b.updateIncomingCommunicationMetrics(n)
+	length := binary.BigEndian.Uint32(header[:4])
+	payload := make([]byte, length-4)
+	n, err = io.ReadFull(b.conn, payload)
+	if err != nil {
+		Logger.Printf("Failed to read SASL handshake payload : %s\n", err.Error())
+		return err
+	}
+	b.updateIncomingCommunicationMetrics(n)
+	res := &SaslHandshakeResponse{}
+	err = versionedDecode(payload, res, 0)
+	if err != nil {
+		Logger.Printf("Failed to parse SASL handshake : %s\n", err.Error())
+		return err
+	}
+	if res.Err != ErrNoError {
+		Logger.Printf("Invalid SASL Mechanism : %s\n", err.Error())
+		return res.Err
+	}
+	Logger.Print("Successful SASL handshake")
+	return nil
+
+}
+
 // Kafka 0.10.0 plans to support SASL Plain and Kerberos as per PR #812 (KIP-43)/(JIRA KAFKA-3149)
 // Some hosted kafka services such as IBM Message Hub already offer SASL/PLAIN auth with Kafka 0.9
 //
@@ -533,6 +580,11 @@ func (b *Broker) responseReceiver() {
 // When credentials are invalid, Kafka closes the connection. This does not seem to be the ideal way
 // of responding to bad credentials but thats how its being done today.
 func (b *Broker) sendAndReceiveSASLPlainAuth() error {
+	handshakeErr := b.sendAndReceiveSASLPlainHandshake()
+	if handshakeErr != nil {
+		Logger.Printf("Error while performing SASL handshake %s\n", b.addr)
+		return handshakeErr
+	}
 	length := 1 + len(b.conf.Net.SASL.User) + 1 + len(b.conf.Net.SASL.Password)
 	authBytes := make([]byte, length+4) //4 byte length header + auth data
 	binary.BigEndian.PutUint32(authBytes, uint32(length))