[#5062] Mark MqttEncoder @Sharable

Motivation:

Commit 2696778 changed MqttEncoder to be a singelton but missed to add @Sharable annotation. This broke the encoder as it can not be added to multiple pipelines.

Modifications:

Add @Sharable annotation

Result:

MqttEncoder can be used in multiple pipelines again.

codec-mqtt/src/main/java/io/netty/handler/codec/mqtt/MqttEncoder.java

@@ -18,6 +18,7 @@ package io.netty.handler.codec.mqtt;
 
 import io.netty.buffer.ByteBuf;
 import io.netty.buffer.ByteBufAllocator;
+import io.netty.channel.ChannelHandler;
 import io.netty.channel.ChannelHandlerContext;
 import io.netty.handler.codec.MessageToMessageEncoder;
 import io.netty.util.CharsetUtil;
@@ -31,6 +32,7 @@ import static io.netty.handler.codec.mqtt.MqttCodecUtil.*;
  * Encodes Mqtt messages into bytes following the protocol specification v3.1
  * as described here <a href="http://public.dhe.ibm.com/software/dw/webservices/ws-mqtt/mqtt-v3r1.html">MQTTV3.1</a>
  */
+@ChannelHandler.Sharable
 public final class MqttEncoder extends MessageToMessageEncoder<MqttMessage> {
 
     public static final MqttEncoder INSTANCE = new MqttEncoder();