Maint: rework publicip package

- Use loopstate package
- Loop interface composition
- Return concrete struct from constructors
- Split into more files
- Add publicip/state package

internal/publicip/state/settings.go

@@ -0,0 +1,39 @@
+package state
+
+import (
+	"context"
+	"reflect"
+
+	"github.com/qdm12/gluetun/internal/configuration"
+)
+
+type SettingsGetterSetter interface {
+	GetSettings() (settings configuration.PublicIP)
+	SetSettings(ctx context.Context,
+		settings configuration.PublicIP) (outcome string)
+}
+
+func (s *State) GetSettings() (settings configuration.PublicIP) {
+	s.settingsMu.RLock()
+	defer s.settingsMu.RUnlock()
+	return s.settings
+}
+
+func (s *State) SetSettings(ctx context.Context, settings configuration.PublicIP) (
+	outcome string) {
+	s.settingsMu.Lock()
+	defer s.settingsMu.Unlock()
+
+	settingsUnchanged := reflect.DeepEqual(s.settings, settings)
+	if settingsUnchanged {
+		return "settings left unchanged"
+	}
+
+	periodChanged := s.settings.Period != settings.Period
+	s.settings = settings
+	if periodChanged {
+		s.updateTicker <- struct{}{}
+		// TODO blocking
+	}
+	return "settings updated"
+}