lib/config, lib/discover: Support new discovery cluster (ref #4618)

This adds one new feature, that discovery servers can have ?nolookup to
be used only for announces. The default set of discovery servers is
changed to:

- discovery.s.n used for lookups. This is dual stack load balanced over
  all discovery servers, and returns both IPv4 and IPV6 results when they
  exist.

- discovery-v4.s.n used for announces. This has IPv4 addresses only and
  the discovery servers will update the unspecified address with the IPv4
  source address, as usual.

- discovery-v6.s.n which is exactly the same for IPv6.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4647

lib/discover/global.go

@@ -30,6 +30,7 @@ type globalClient struct {
 	announceClient httpClient
 	queryClient    httpClient
 	noAnnounce     bool
+	noLookup       bool
 	stop           chan struct{}
 	errorHolder
 }
@@ -52,6 +53,7 @@ type announcement struct {
 type serverOptions struct {
 	insecure   bool   // don't check certificate
 	noAnnounce bool   // don't announce
+	noLookup   bool   // don't use for lookups
 	id         string // expected server device ID
 }
 
@@ -119,15 +121,26 @@ func NewGlobal(server string, cert tls.Certificate, addrList AddressLister) (Fin
 		announceClient: announceClient,
 		queryClient:    queryClient,
 		noAnnounce:     opts.noAnnounce,
+		noLookup:       opts.noLookup,
 		stop:           make(chan struct{}),
 	}
-	cl.setError(errors.New("not announced"))
+	if !opts.noAnnounce {
+		// If we are supposed to annonce, it's an error until we've done so.
+		cl.setError(errors.New("not announced"))
+	}
 
 	return cl, nil
 }
 
 // Lookup returns the list of addresses where the given device is available
 func (c *globalClient) Lookup(device protocol.DeviceID) (addresses []string, err error) {
+	if c.noLookup {
+		return nil, lookupError{
+			error:    errors.New("lookups not supported"),
+			cacheFor: time.Hour,
+		}
+	}
+
 	qURL, err := url.Parse(c.server)
 	if err != nil {
 		return nil, err
@@ -201,7 +214,6 @@ func (c *globalClient) Serve() {
 }
 
 func (c *globalClient) sendAnnouncement(timer *time.Timer) {
-
 	var ann announcement
 	if c.addrList != nil {
 		ann.Addresses = c.addrList.ExternalAddresses()
@@ -287,6 +299,7 @@ func parseOptions(dsn string) (server string, opts serverOptions, err error) {
 	opts.id = q.Get("id")
 	opts.insecure = opts.id != "" || queryBool(q, "insecure")
 	opts.noAnnounce = queryBool(q, "noannounce")
+	opts.noLookup = queryBool(q, "nolookup")
 
 	// Check for disallowed combinations
 	if p.Scheme == "http" {