p2p/nat: limit UPNP request concurrency (#21390)

This adds a lock around requests because some routers can't handle
concurrent requests. Requests are also rate-limited.
 
The Map function request a new mapping exactly when the map timeout
occurs instead of 5 minutes earlier. This should prevent duplicate mappings.

p2p/nat/nat.go

@@ -91,15 +91,14 @@ func Parse(spec string) (Interface, error) {
 }
 
 const (
-	mapTimeout        = 20 * time.Minute
-	mapUpdateInterval = 15 * time.Minute
+	mapTimeout = 10 * time.Minute
 )
 
 // Map adds a port mapping on m and keeps it alive until c is closed.
 // This function is typically invoked in its own goroutine.
-func Map(m Interface, c chan struct{}, protocol string, extport, intport int, name string) {
+func Map(m Interface, c <-chan struct{}, protocol string, extport, intport int, name string) {
 	log := log.New("proto", protocol, "extport", extport, "intport", intport, "interface", m)
-	refresh := time.NewTimer(mapUpdateInterval)
+	refresh := time.NewTimer(mapTimeout)
 	defer func() {
 		refresh.Stop()
 		log.Debug("Deleting port mapping")
@@ -121,7 +120,7 @@ func Map(m Interface, c chan struct{}, protocol string, extport, intport int, na
 			if err := m.AddMapping(protocol, extport, intport, name, mapTimeout); err != nil {
 				log.Debug("Couldn't add port mapping", "err", err)
 			}
-			refresh.Reset(mapUpdateInterval)
+			refresh.Reset(mapTimeout)
 		}
 	}
 }