Maint: dns package state rework

- Interface composition with loopstate interfaces
- Use loopstate.Manager
- Create dns/state package for handling settings

internal/dns/state/state.go

@@ -0,0 +1,33 @@
+package state
+
+import (
+	"sync"
+
+	"github.com/qdm12/gluetun/internal/configuration"
+	"github.com/qdm12/gluetun/internal/loopstate"
+)
+
+var _ Manager = (*State)(nil)
+
+type Manager interface {
+	SettingsGetterSetter
+}
+
+func New(statusApplier loopstate.Applier,
+	settings configuration.DNS,
+	updateTicker chan<- struct{}) *State {
+	return &State{
+		statusApplier: statusApplier,
+		settings:      settings,
+		updateTicker:  updateTicker,
+	}
+}
+
+type State struct {
+	statusApplier loopstate.Applier
+
+	settings   configuration.DNS
+	settingsMu sync.RWMutex
+
+	updateTicker chan<- struct{}
+}