Loops and HTTP control server rework (#308)

- CRUD REST HTTP server
- `/v1` HTTP server prefix
- Retrocompatible with older routes (redirects to v1 or handles the requests directly)
- DNS, Updater and Openvpn refactored to have a REST-like state with new methods to change their states synchronously
- Openvpn, Unbound and Updater status, see #287

internal/constants/status.go

@@ -0,0 +1,14 @@
+package constants
+
+import (
+	"github.com/qdm12/gluetun/internal/models"
+)
+
+const (
+	Starting  models.LoopStatus = "starting"
+	Running   models.LoopStatus = "running"
+	Stopping  models.LoopStatus = "stopping"
+	Stopped   models.LoopStatus = "stopped"
+	Crashed   models.LoopStatus = "crashed"
+	Completed models.LoopStatus = "completed"
+)