Bubble up an event when Bob receives message 3 response

Before this patch Bob is not sending message 3. This is because we are not
polling Bob's swarm correctly. To fix it we can just mimic the other NB's and
bubble up an event when Bob receives message 3 response from Alice, this way we
can `await` upon this event which triggers polling, making Bob's swarm send the
message.

xmr-btc/src/bob.rs

@@ -152,6 +152,8 @@ where
                     Either::Right(_) => return Err(SwapFailed::AfterBtcLock(Reason::BtcExpired)),
                 };
 
+                tracing::debug!("select returned transfer proof from message");
+
                 transfer_proof
             };