Client-side timeout of RPCs

[oberstet]

Provide variant 1 of the following 3 in AutobahnJS:

1. timeout/reject the promise client-side without telling the server, and should the server respond later, silently ignore the response (which might be a success or error response)
2. timeout/reject the promise client-side and actively cancel the call
3. call server with timeout communicated. timeout call execution on server-side, and return error ("call timeout")