On Read Retransmit send FSM to SENDING

RFC6347 Section-4.2.4 states

```
The implementation reads a retransmitted flight from the peer: the
implementation transitions to the SENDING state, where it
retransmits the flight, resets the retransmit timer, and returns
to the WAITING state.  The rationale here is that the receipt of a
duplicate message is the likely result of timer expiry on the peer
and therefore suggests that part of one's previous flight was
lost.
```

Resolves #478

handshaker.go

@@ -294,13 +294,18 @@ func (s *handshakeFSM) wait(ctx context.Context, c flightConn) (handshakeState,
 			if err != nil {
 				return handshakeErrored, err
 			}
+
+			s.cfg.log.Tracef("[handshake:%s] %s -> %s", srvCliStr(s.state.isClient), s.currentFlight.String(), nextFlight.String())
 			if nextFlight == 0 {
-				break
+				if s.currentFlight == flight5 {
+					break
+				}
+				return handshakeSending, nil
 			}
-			s.cfg.log.Tracef("[handshake:%s] %s -> %s", srvCliStr(s.state.isClient), s.currentFlight.String(), nextFlight.String())
 			if nextFlight.isLastRecvFlight() && s.currentFlight == nextFlight {
 				return handshakeFinished, nil
 			}
+
 			s.currentFlight = nextFlight
 			return handshakePreparing, nil