Use ThreadLocalRandom instead of Math.random() (#11165)

Motivation:

`ThreadLocalRandom` doesn't cause contention. Also `nextInt()` generates only 4 random bytes while `Math.random()` generates 8 bytes.

Modification:

Replaced `(int) Math.random()` with `PlatformDependent.threadLocalRandom().nextInt()`

Result:

No possible contention when random numbers for WebSockets.

codec-http/src/main/java/io/netty/handler/codec/http/websocketx/WebSocket08FrameEncoder.java

@@ -57,6 +57,7 @@
 import io.netty.channel.ChannelHandlerContext;
 import io.netty.handler.codec.MessageToMessageEncoder;
 import io.netty.handler.codec.TooLongFrameException;
+import io.netty.util.internal.PlatformDependent;
 import io.netty.util.internal.logging.InternalLogger;
 import io.netty.util.internal.logging.InternalLoggerFactory;
 
@@ -178,7 +179,7 @@ protected void encode(ChannelHandlerContext ctx, WebSocketFrame msg, List<Object
 
             // Write payload
             if (maskPayload) {
-                int random = (int) (Math.random() * Integer.MAX_VALUE);
+                int random = PlatformDependent.threadLocalRandom().nextInt(Integer.MAX_VALUE);
                 mask = ByteBuffer.allocate(4).putInt(random).array();
                 buf.writeBytes(mask);
 

codec-http/src/main/java/io/netty/handler/codec/http/websocketx/WebSocketClientHandshaker00.java

@@ -26,6 +26,7 @@
 import io.netty.handler.codec.http.HttpMethod;
 import io.netty.handler.codec.http.HttpResponseStatus;
 import io.netty.handler.codec.http.HttpVersion;
+import io.netty.util.internal.PlatformDependent;
 
 import java.net.URI;
 import java.nio.ByteBuffer;
@@ -254,7 +255,7 @@ private static String insertRandomCharacters(String key) {
         char[] randomChars = new char[count];
         int randCount = 0;
         while (randCount < count) {
-            int rand = (int) (Math.random() * 0x7e + 0x21);
+            int rand = PlatformDependent.threadLocalRandom().nextInt(0x7e) + 0x21;
             if (0x21 < rand && rand < 0x2f || 0x3a < rand && rand < 0x7e) {
                 randomChars[randCount] = (char) rand;
                 randCount += 1;