Persistent server pools (#226)

* GetAllServers with version & timestamp tests
* Storage package to sync servers
* Use storage Sync to get and use servers

internal/storage/storage.go

@@ -0,0 +1,29 @@
+package storage
+
+import (
+	"io/ioutil"
+	"os"
+
+	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/golibs/logging"
+)
+
+type Storage interface {
+	SyncServers(hardcodedServers models.AllServers) (allServers models.AllServers, err error)
+}
+
+type storage struct {
+	osStat    func(name string) (os.FileInfo, error)
+	readFile  func(filename string) (data []byte, err error)
+	writeFile func(filename string, data []byte, perm os.FileMode) error
+	logger    logging.Logger
+}
+
+func New(logger logging.Logger) Storage {
+	return &storage{
+		osStat:    os.Stat,
+		readFile:  ioutil.ReadFile,
+		writeFile: ioutil.WriteFile,
+		logger:    logger.WithPrefix("storage: "),
+	}
+}