std/net/wamp: open websocket in the client thread, use tgroups
std/net/wamp: open websocket in the client thread, use tgroups

In order to make sure the websocket threads are in the right thread group,
we need to start the websocket client from the wamp client thread.
We use an asynchronous completion token to unblock our client.

vyzo/gerbil,vyzo/gerbil,vyzo/gerbil