Implement basic retransmit queue reset method (#52)

* implement basic retransmit queue reset method

* tcp: rto reset on new ack; accept ack after retransmit bugfix

tcp/handler_test.go

@@ -53,6 +53,7 @@ func newHandler(t *testing.T, mtu, mintaxpackets int) *Handler {
 	if err != nil {
 		t.Fatal(err)
 	}
+	h.rto = rtoInitial // Fake time: now=0 and sentAt=0, so RTO never fires in tests.
 	return h
 }