Thread race condition

[qiuzhong]

As rclnodejs is the dependency of ros2-web-bridge, there is thread race condition which may lead the ros2-web-bridge server crash.

There are already some tricky issues in ros2-web-bridge, like #67, #69. And we suspect they're all caused by this thread race condition.

[minggangw]

I am going to fix this, thanks!