The new Setup.Stop RPC allows clients to abort long-running operations.
Each operation implementation must add itself to the server's stopper,
and listen on the given channel for a signal to abort.
In this commit, plan, apply, and validate operations translate the stop
signal into a context cancellation. This alone is not yet sufficient for
a truly graceful shutdown, so in later commits we need to adjust the
stacks runtime to instruct the modules runtime to stop on cancellation.