BUG description
When the websocket server is started, many background worker thread is created. On my PC, it is 21. I accept this. But the problem comes with the websocket usage. Every time we communicate through the websocket, the background worker threads number is increasing to a given amount. But when the communication ends (the websocket stays still connected), these threads are not reduced fully to the original amount.

From this problem after a while our server has more than 600 threads and huge memory usage.

To Reproduce
Steps to reproduce the behavior:

1. Start websocket server
2. Connect to the server
3. Send a message from client, do something on the server and send a response (we send json messages)
4. During this time you can check the number of threads like this: ps -eLf | grep JavaWebsocketServer | wc -l
5. Repeat the 3. step

Environment:

• Version used: 1.6
• Java version: OpenJDK Runtime Environment (build 17.0.14+7)
• Operating System and version: Debian Linux and Linux version 6.12.20-2-MANJARO