start_conversation without expecting a reply?

Right now, when we send a RENDEZVOUS1 message, we expect no reply from the rendezvous point. But the start_conversation() API forces us to install a message handler that will (in the happy case) never be triggered.

Should we have a send_raw_msg_to_hop API that bypasses conversations entirely? Or should we have some way to say "no reply is expected" in the start_conversation API?

Found while working on #980 (closed).

cc @gabi-250