Send a control packet immediately on session state change

RFC 5880 6.8.7 says a packet SHOULD be sent between periodic transmissions when its contents would change, so peers see state changes without waiting up to a full interval.

src/bin/rust-bfdd.rs

@@ -545,6 +545,10 @@ impl BfdSession {
             _ => {}
         }
         self.state.set_session_state(new_state);
+        // https://datatracker.ietf.org/doc/html/rfc5880#section-6.8.7
+        //   A BFD Control packet SHOULD be transmitted during the interval between periodic Control packet
+        //   transmissions when the contents of that packet would differ from that in the previously transmitted packet
+        self.transmit_control_packet(false).await;
     }
 
     async fn run(&mut self) {