fix(dht): `WebsocketServer` memory leak

[teogeb]

Improve WebsocketServer cleanup. When we explicitly remove httpServer listeners, the instance doesn't leak memory immediately after WebsocketServer#stop is called.

There wasn't a permanent memory leak: e.g. await wait(100) before the check in the unit test would also make the test pass.

Open questions

Why the change causes this tests to fail:

dht-browser:

1) starts and stops
     WebsocketServer
     ReferenceError: require is not defined