`Chain.Verify()` fails with mixture of Roughtime versions

[cjpatton]

To reproduce:

Start a couple of Roughtime servers:

$ go run ./cmd/testserver -addr 127.0.0.1:2002
main.go:64: Root public key: sm2tho24GOkdU26vpQInrruJKumCEDLCVHTArPBPqDY=

go run ./cmd/testserver -addr 127.0.0.1:2003
main.go:64: Root public key: bPDagpXQCofeGLeu8GwrX2CQ5Udy8rk1ziiFJGowZIw=

Then create a configuration file for which the client will try IETF-Roughitme for one server but not the other:

{
  "servers": [
    {
      "name": "server1",
      "version": "IETF-Roughtime",
      "publicKeyType": "ed25519",
      "publicKey": "sm2tho24GOkdU26vpQInrruJKumCEDLCVHTArPBPqDY=",
      "addresses": [
        {
          "protocol": "udp",
          "address": "127.0.0.1:2002"
        }
      ]
    },
    {
      "name": "server2",
      "publicKeyType": "ed25519",
      "publicKey": "bPDagpXQCofeGLeu8GwrX2CQ5Udy8rk1ziiFJGowZIw=",
      "addresses": [
        {
          "protocol": "udp",
          "address": "127.0.0.1:2003"
        }
      ]
    }
  ]
}

Finally, test this as follows:

package main

import (
	"testing"

	"github.com/cloudflare/roughtime/client"
)

func TestRoughtimeServer(t *testing.T) {
	results, err := client.DoFromFile("devdata/config/client.config", client.DefaultQueryAttempts, client.DefaultQueryTimeout, nil)
	if err != nil {
		t.Fatal(err)
	}

	chain := client.NewChain(results)
	if chain == nil {
		t.Fatal("chain  == nil, want chin != nil")
	}

	ok, err := chain.Verify(nil)
	if err != nil {
		t.Errorf("verification fails: %s", err)
	} else if !ok {
		t.Error("chain not valid, want valid")
	}
}

You should see something like:

$ go test client_test.go
--- FAIL: TestRoughtimeServer (0.00s)
    client_test.go:24: verification fails: missing VER tag
FAIL
FAIL	command-line-arguments	0.407s
FAIL

It seems to me that it should be valid to chain together signed timestamps across versions, but this requires a bit of thought.