rpc: use cancel signal in RecvMessage
rpc: use cancel signal in RecvMessage

Failing to listen to the cancel signal will cause a connection to
deadlock on close.  Hopefully, most io.ReadWriteCloser implementations
will interrupt the Read on Close.

zombiezen/go-capnproto2,zombiezen/go-capnproto2,zombiezen/go-capnproto,hodduc/go-capnproto2,zombiezen/go-capnproto,hodduc/go-capnproto2