Workaround for lifetime poisoning with client?

I am looking into implementing call canceling. My first idea to minimize the impact of the changes was to spawn a task for the client to wait for call results, and return a cancelable future from `Client::call` that would abort the task (if required based on options) and send the WAMP `CANCEL` message using the Client. This doesn't appear to be possible because of the lifetime poisoning that occurs as described in the commit message from #5.

Anyone have another idea how this could be implemented. The main issue is that you need the call `request_id` to cancel the call, but in the current implementation of `Client::call` you wouldn't have the `request_id` until receiving the result.

The only thing I can think is that `Client::call` only returns a struct with the `request_id`, and an implementation which would allow you to get the `results` future, or execute `cancel` from there.