Reuse the same cookie until it expires

This fixes and issue where the client retries and fails because the cookie has been changed.

transport-raknet/src/main/java/org/cloudburstmc/netty/handler/codec/raknet/server/RakServerOfflineHandler.java

@@ -183,7 +183,7 @@ private void onOpenConnectionRequest1(ChannelHandlerContext ctx, DatagramPacket
             cookie = 0;
         }
 
-        PendingConnection connection = this.pendingConnections.put(sender, new PendingConnection(protocolVersion, cookie));
+        PendingConnection connection = this.pendingConnections.putIfAbsent(sender, new PendingConnection(protocolVersion, cookie));
         if (connection != null && log.isTraceEnabled()) {
             log.trace("Received duplicate open connection request 1 from {}", sender);
         }