Heartbeat skip serialize and deserialize

[zhangyz-hd]

What is the purpose of the change

提供者和消费者之间的心跳，Request和Response的Data都为null：

//org.apache.dubbo.remoting.exchange.support.header.HeartbeatTimerTask#doTask
Request req = new Request();
req.setVersion(Version.getProtocolVersion());
req.setTwoWay(true);
req.setEvent(HEARTBEAT_EVENT);//HEARTBEAT_EVENT = null

//org.apache.dubbo.remoting.exchange.support.header.HeartbeatHandler#received
Response res = new Response(req.getId(), req.getVersion());
res.setEvent(HEARTBEAT_EVENT);//HEARTBEAT_EVENT = null

当提供者和消费者数量非常大时，每个节点都需要发送/接受大量的心跳Requet和Response，这些Requet、Response也会和服务调用一样经过serialize+encode、decode+deserialize，而其中的serialize/deserialize需要消耗过多的CPU资源。
这个优化的目的是对于心跳Requet/Response跳过serialize/deserialize处理，serialize改为赋值为null对象的序列化后字节，deserialize改为直接赋值null。

Brief changelog

在CodecSupport中增加getNullBytesOf(Serialization s)方法，首次访问时根据serialization获得null对象的字节数组并缓存到ID_NULLBYTES_MAP
DubboCodec、ExchangeCodec中在调整encodeRequest、encodeResponse、decodeBody逻辑，实现上述目标。

备注：目前DUBBO的Serialization的实现中，以下三个实现其实尚不支持null对象的序列化，在CodecSupport.getNullBytesOf中降级为返回空字符串。

1. KryoSerialization2
2. GenericProtobufSerialization
3. GenericProtobufJsonSerialization

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[codecov-io]

Codecov Report

Merging #7077 (fdc767a) into master (70b6fb3) will increase coverage by 0.38%.
The diff coverage is 63.07%.

@@             Coverage Diff              @@
##             master    #7077      +/-   ##
============================================
+ Coverage     59.22%   59.61%   +0.38%     
+ Complexity      508      288     -220     
============================================
  Files          1028     1001      -27     
  Lines         41525    39994    -1531     
  Branches       6038     5941      -97     
============================================
- Hits          24593    23841     -752     
+ Misses        14169    13440     -729     
+ Partials       2763     2713      -50     

Impacted Files	Coverage Δ	Complexity Δ
...rg/apache/dubbo/rpc/protocol/dubbo/DubboCodec.java	61.32% <30.00%> (-2.68%)	0.00 <0.00> (ø)
...e/dubbo/remoting/exchange/codec/ExchangeCodec.java	73.33% <57.14%> (-0.97%)	0.00 <0.00> (ø)
.../apache/dubbo/remoting/transport/CodecSupport.java	82.00% <90.00%> (+5.33%)	0.00 <0.00> (ø)
...che/dubbo/common/utils/CharSequenceComparator.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (ø%)
...che/dubbo/rpc/cluster/support/FailfastCluster.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (ø%)
...bo/rpc/cluster/interceptor/ClusterInterceptor.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (ø%)
...java/org/apache/dubbo/remoting/etcd/Constants.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (ø%)
...pache/dubbo/metadata/report/support/Constants.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (ø%)
...igurator/override/OverrideConfiguratorFactory.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (ø%)
...e/dubbo/registry/status/RegistryStatusChecker.java	0.00% <0.00%> (-81.25%)	0.00% <0.00%> (ø%)
... and 182 more

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[AlbumenJ]

LGTM.