[More Optimize]Heartbeat skip serialize and deserialize (apache#7168)

* 对于心跳的Request、Response，序列化时获取已缓存的null对象的序列化后字节，反序列化时则赋值null，以降低心跳对CPU带来的开销。

* rename method getNullBytes(s) -> getNullBytesOf(s)

* format code

* Use byte array comparison to identify the event is heartbeats

* Use NIO-ThreadLocal buffer to read event payload, to reduce mem cost.

dubbo-remoting/dubbo-remoting-api/src/main/java/org/apache/dubbo/remoting/transport/CodecSupport.java

@@ -46,6 +46,8 @@ public class CodecSupport {
     private static Map<String, Byte> SERIALIZATIONNAME_ID_MAP = new HashMap<String, Byte>();
     // Cache null object serialize results, for heartbeat request/response serialize use.
     private static Map<Byte, byte[]> ID_NULLBYTES_MAP = new HashMap<Byte, byte[]>();
+    // NIO ThreadLocal buffer to read event payload
+    private static final ThreadLocal<byte[]> TL_BUFFER = ThreadLocal.withInitial(() -> new byte[1024]);
 
     static {
         Set<String> supportedExtensions = ExtensionLoader.getExtensionLoader(Serialization.class).getSupportedExtensions();
@@ -132,7 +134,7 @@ public static byte[] getNullBytesOf(Serialization s) {
      */
     public static byte[] getPayload(InputStream is) throws IOException {
         ByteArrayOutputStream baos = new ByteArrayOutputStream();
-        byte[] buffer = new byte[1024];
+        byte[] buffer = getBuffer(is.available());
         int len;
         while ((len = is.read(buffer)) > -1) {
             baos.write(buffer, 0, len);
@@ -141,6 +143,14 @@ public static byte[] getPayload(InputStream is) throws IOException {
         return baos.toByteArray();
     }
 
+    private static byte[] getBuffer(int size) {
+        byte[] bytes = TL_BUFFER.get();
+        if (size <= bytes.length) {
+            return bytes;
+        }
+        return new byte[size];
+    }
+
     /**
      * Check if payload is null object serialize result byte[] of serialization
      *