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

conn.go

@@ -878,21 +878,32 @@ func (c *Conn) handleIncomingPacket(ctx context.Context, buf []byte, rAddr net.A
 		}
 	}
 
-	isHandshake, err := c.fragmentBuffer.push(append([]byte{}, buf...))
+	isHandshake, isRetransmit, err := c.fragmentBuffer.push(append([]byte{}, buf...))
 	if err != nil {
 		// Decode error must be silently discarded
 		// [RFC6347 Section-4.1.2.7]
 		c.log.Debugf("defragment failed: %s", err)
 		return false, nil, nil
 	} else if isHandshake {
 		markPacketAsValid()
-		for out, epoch := c.fragmentBuffer.pop(); out != nil; out, epoch = c.fragmentBuffer.pop() {
-			header := &handshake.Header{}
-			if err := header.Unmarshal(out); err != nil {
-				c.log.Debugf("%s: handshake parse failed: %s", srvCliStr(c.state.isClient), err)
-				continue
+
+		if isRetransmit {
+			// The implementation reads a retransmitted flight from the peer: the
+			// implementation transitions to the SENDING state
+			// [RFC6347 Section-4.2.4]
+			select {
+			case c.fsm.readRetransmit <- struct{}{}:
+			default:
+			}
+		} else {
+			for out, epoch := c.fragmentBuffer.pop(); out != nil; out, epoch = c.fragmentBuffer.pop() {
+				header := &handshake.Header{}
+				if err := header.Unmarshal(out); err != nil {
+					c.log.Debugf("%s: handshake parse failed: %s", srvCliStr(c.state.isClient), err)
+					continue
+				}
+				c.handshakeCache.push(out, epoch, header.MessageSequence, header.Type, !c.state.isClient)
 			}
-			c.handshakeCache.push(out, epoch, header.MessageSequence, header.Type, !c.state.isClient)
 		}
 
 		return true, nil, nil

fragment_buffer.go

@@ -43,24 +43,29 @@ func (f *fragmentBuffer) size() int {
 // Attempts to push a DTLS packet to the fragmentBuffer
 // when it returns true it means the fragmentBuffer has inserted and the buffer shouldn't be handled
 // when an error returns it is fatal, and the DTLS connection should be stopped
-func (f *fragmentBuffer) push(buf []byte) (bool, error) {
+func (f *fragmentBuffer) push(buf []byte) (isHandshake, isRetransmit bool, err error) {
 	if f.size()+len(buf) >= fragmentBufferMaxSize {
-		return false, errFragmentBufferOverflow
+		return false, false, errFragmentBufferOverflow
 	}
 
 	frag := new(fragment)
 	if err := frag.recordLayerHeader.Unmarshal(buf); err != nil {
-		return false, err
+		return false, false, err
 	}
 
 	// fragment isn't a handshake, we don't need to handle it
 	if frag.recordLayerHeader.ContentType != protocol.ContentTypeHandshake {
-		return false, nil
+		return false, false, nil
 	}
 
 	for buf = buf[recordlayer.FixedHeaderSize:]; len(buf) != 0; frag = new(fragment) {
 		if err := frag.handshakeHeader.Unmarshal(buf); err != nil {
-			return false, err
+			return false, false, err
+		}
+
+		// Fragment is a retransmission, we have already assembled it so ignoring
+		if frag.handshakeHeader.MessageSequence < f.currentMessageSequenceNumber {
+			return true, true, nil
 		}
 
 		if _, ok := f.cache[frag.handshakeHeader.MessageSequence]; !ok {
@@ -80,7 +85,7 @@ func (f *fragmentBuffer) push(buf []byte) (bool, error) {
 		buf = buf[end:]
 	}
 
-	return true, nil
+	return true, false, nil
 }
 
 func (f *fragmentBuffer) pop() (content []byte, epoch uint16) {

handshaker.go

@@ -90,6 +90,7 @@ type handshakeFSM struct {
 	cache              *handshakeCache
 	cfg                *handshakeConfig
 	closed             chan struct{}
+	readRetransmit     chan struct{}
 }
 
 type handshakeConfig struct {
@@ -173,6 +174,7 @@ func newHandshakeFSM(
 		cfg:                cfg,
 		retransmitInterval: cfg.initialRetransmitInterval,
 		closed:             make(chan struct{}),
+		readRetransmit:     make(chan struct{}, 1),
 	}
 }
 
@@ -303,7 +305,6 @@ func (s *handshakeFSM) wait(ctx context.Context, c flightConn) (handshakeState,
 			}
 			s.currentFlight = nextFlight
 			return handshakePreparing, nil
-
 		case <-retransmitTimer.C:
 			if !s.retransmit {
 				return handshakeWaiting, nil
@@ -319,6 +320,8 @@ func (s *handshakeFSM) wait(ctx context.Context, c flightConn) (handshakeState,
 				s.retransmitInterval = time.Second * 60
 			}
 			return handshakeSending, nil
+		case <-s.readRetransmit:
+			return handshakeSending, nil
 		case <-ctx.Done():
 			s.retransmitInterval = s.cfg.initialRetransmitInterval
 			return handshakeErrored, ctx.Err()