Persistent server pools (#226)

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

internal/constants/servers.go

@@ -0,0 +1,49 @@
+package constants
+
+import "github.com/qdm12/gluetun/internal/models"
+
+func GetAllServers() (allServers models.AllServers) {
+	return models.AllServers{
+		Version: 1, // used for migration of the top level scheme
+		Cyberghost: models.CyberghostServers{
+			Version:   1,          // model version
+			Timestamp: 1598236838, // latest takes precedence
+			Servers:   CyberghostServers(),
+		},
+		Mullvad: models.MullvadServers{
+			Version:   1,
+			Timestamp: 1598236838,
+			Servers:   MullvadServers(),
+		},
+		Nordvpn: models.NordvpnServers{
+			Version:   1,
+			Timestamp: 1598236838,
+			Servers:   NordvpnServers(),
+		},
+		Pia: models.PiaServers{
+			Version:   1,
+			Timestamp: 1598236838,
+			Servers:   PIAServers(),
+		},
+		Purevpn: models.PurevpnServers{
+			Version:   1,
+			Timestamp: 1598236838,
+			Servers:   PurevpnServers(),
+		},
+		Surfshark: models.SurfsharkServers{
+			Version:   1,
+			Timestamp: 1598236838,
+			Servers:   SurfsharkServers(),
+		},
+		Vyprvpn: models.VyprvpnServers{
+			Version:   1,
+			Timestamp: 1598236838,
+			Servers:   VyprvpnServers(),
+		},
+		Windscribe: models.WindscribeServers{
+			Version:   1,
+			Timestamp: 1598236838,
+			Servers:   WindscribeServers(),
+		},
+	}
+}