Replace the type of fragments field with IntObjectMap in SctpMessageC…

…ompletionHandler (#10057)

Motivation:

The type IntObjectMap with the key of primitive int can help lower the cost of boxing and unboxing,  and improve performance. I think it's more suitable for fragments field in SctpMessageCompletionHandler.java.

Modification:

Just replace the type of fragments field with IntObjectMap.

Result:
Improve performance slightly while decoding sctp message.

transport-sctp/src/main/java/io/netty/handler/codec/sctp/SctpMessageCompletionHandler.java

@@ -22,18 +22,18 @@
 import io.netty.channel.ChannelInboundHandler;
 import io.netty.channel.sctp.SctpMessage;
 import io.netty.handler.codec.MessageToMessageDecoder;
+import io.netty.util.collection.IntObjectHashMap;
+import io.netty.util.collection.IntObjectMap;
 
-import java.util.HashMap;
 import java.util.List;
-import java.util.Map;
 
 /**
  * {@link MessageToMessageDecoder} which will take care of handle fragmented {@link SctpMessage}s, so
  * only <strong>complete</strong> {@link SctpMessage}s will be forwarded to the next
  * {@link ChannelInboundHandler}.
  */
 public class SctpMessageCompletionHandler extends MessageToMessageDecoder<SctpMessage> {
-    private final Map<Integer, ByteBuf> fragments = new HashMap<Integer, ByteBuf>();
+    private final IntObjectMap<ByteBuf> fragments = new IntObjectHashMap<ByteBuf>();
 
     @Override
     protected void decode(ChannelHandlerContext ctx, SctpMessage msg, List<Object> out) throws Exception {