[JS] Unravel the WebSocket implementation

[ffendt]

The WebSocket implementation of the client API currently is very difficult to read, understand and extend. When reading the code (or the tests for it), it is not possible to understand if everything works as expected or if it doesn't.

Therefore the implementation definitely requires some work (and probably a rewrite?).

Additionally to the existing provided functionality, there should also be the possibility to manually connect and disconnect the WebSocket client. Currently it will connect instantly after building it and there is no way to disconnect it.