Fix DTLS client role in long delay connections

Fixes pion/webrtc#2089

When a retranmission from the remote side arrives after
the handshake is complete, the `finish` routine
puts it back into retransmit loop.

With Chrome, this fails after 15 seconds.
Firefox does not error out though.

AUTHORS.txt

@@ -9,6 +9,7 @@ Arlo Breault <[email protected]>
 Atsushi Watanabe <[email protected]>
 backkem <[email protected]>
 bjdgyc <[email protected]>
+boks1971 <[email protected]>
 Bragadeesh <[email protected]>
 Carson Hoffman <[email protected]>
 Cecylia Bocovich <[email protected]>

handshaker.go

@@ -326,6 +326,9 @@ func (s *handshakeFSM) finish(ctx context.Context, c flightConn) (handshakeState
 		if nextFlight == 0 {
 			break
 		}
+		if s.currentFlight == nextFlight && (nextFlight.isLastRecvFlight() || nextFlight.isLastSendFlight()) {
+			return handshakeFinished, nil
+		}
 		<-retransmitTimer.C
 		// Retransmit last flight
 		return handshakeSending, nil