Limit how much of seeder response may be decoded

A limit of 4096 bytes is imposed on the response body, of which the
first 16 results must fit.  This leaves room to add more fields to the
seeder response objects without being too large, while limiting
attacks from malicious seeders that send non-terminating json object.

While here, improve the log messages to report which seeder any errors
occurred for.

p2p/peering.go

@@ -300,11 +300,12 @@ func (lp *LocalPeer) SeedPeers(ctx context.Context, services wire.ServiceFlag) {
 		if resp == nil {
 			continue
 		}
+		seeder := resp.Request.Host
 		var apiResponse struct {
 			Host     string `json:"host"`
 			Services uint64 `json:"services"`
 		}
-		dec := json.NewDecoder(resp.Body)
+		dec := json.NewDecoder(io.LimitReader(resp.Body, 4096))
 		na = na[:0]
 		// Read at most 16 entries from each seeder, discard rest
 		for i := 0; i < 16; i++ {
@@ -313,25 +314,28 @@ func (lp *LocalPeer) SeedPeers(ctx context.Context, services wire.ServiceFlag) {
 				break
 			}
 			if err != nil {
-				log.Warnf("Invalid seeder API response: %v", err)
-				continue
+				log.Warnf("Invalid seeder %v API response: %v", seeder, err)
+				break
 			}
 			host, port, err := net.SplitHostPort(apiResponse.Host)
 			if err != nil {
-				log.Warnf("Invalid host in seeder API: %v", err)
+				log.Warnf("Invalid host in seeder %v API: %v", seeder, err)
 				continue
 			}
 			ip := net.ParseIP(host)
 			if ip == nil {
-				log.Warnf("Invalid IP address %q in seeder API host field", host)
+				log.Warnf("Invalid IP address %q in seeder %v API host field",
+					host, seeder)
 				continue
 			}
 			portNum, err := strconv.ParseUint(port, 10, 16)
 			if err != nil {
-				log.Warnf("Invalid port %q in seeder API host field", port)
+				log.Warnf("Invalid port %q in seeder %v API host field", port,
+					seeder)
 				continue
 			}
-			log.Debugf("Discovered peer %v from seeder", apiResponse.Host)
+			log.Debugf("Discovered peer %v from seeder %v", apiResponse.Host,
+				seeder)
 			na = append(na, &wire.NetAddress{
 				Timestamp: time.Now(),
 				Services:  wire.ServiceFlag(apiResponse.Services),