Refactor config commit stuff to support restartless updates better

Includes restartless updates of the GUI settings (listening port etc) as
a proof of concept.

cmd/syncthing/connections.go

@@ -353,3 +353,24 @@ func (s *connectionSvc) shouldLimit(addr net.Addr) bool {
 	}
 	return !tcpaddr.IP.IsLoopback()
 }
+
+func (s *connectionSvc) VerifyConfiguration(from, to config.Configuration) error {
+	return nil
+}
+
+func (s *connectionSvc) CommitConfiguration(from, to config.Configuration) bool {
+	// We require a restart if a device as been removed.
+
+	newDevices := make(map[protocol.DeviceID]bool, len(to.Devices))
+	for _, dev := range to.Devices {
+		newDevices[dev.DeviceID] = true
+	}
+
+	for _, dev := range from.Devices {
+		if !newDevices[dev.DeviceID] {
+			return false
+		}
+	}
+
+	return true
+}