server: add periodic renewal of port forwarding

The check prevented the creation of port forwardings which were assumed
to be present already. After this change the port forwardings which
might have been removed from the NAT device can be re-created.

nat/pmp.go

@@ -65,10 +65,6 @@ func (p *PMP) AddPortMapping(port uint16) error {
 	p.forwardedPortsMtx.Lock()
 	defer p.forwardedPortsMtx.Unlock()
 
-	if _, exists := p.forwardedPorts[port]; exists {
-		return nil
-	}
-
 	_, err := p.client.AddPortMapping("tcp", int(port), int(port), 0)
 	if err != nil {
 		return err

nat/upnp.go

@@ -62,10 +62,6 @@ func (u *UPnP) AddPortMapping(port uint16) error {
 	u.forwardedPortsMtx.Lock()
 	defer u.forwardedPortsMtx.Unlock()
 
-	if _, exists := u.forwardedPorts[port]; exists {
-		return nil
-	}
-
 	if err := u.device.Forward(port, ""); err != nil {
 		return err
 	}