Maint: internal/storage rework

- No more global variables
- Inject merged servers to configuration package
- Fix #566: configuration parsing to use persisted servers.json
- Move server data files from `internal/constants` to `internal/storage`

internal/provider/privateinternetaccess/portforward.go

@@ -33,7 +33,7 @@ var (
 func (p *PIA) PortForward(ctx context.Context, client *http.Client,
 	logger logging.Logger, gateway net.IP, serverName string) (
 	port uint16, err error) {
-	server := constants.PIAServerWhereName(serverName)
+	server := constants.PIAServerWhereName(p.servers, serverName)
 	if !server.PortForward {
 		logger.Error("The server " + serverName +
 			" (region " + server.Region + ") does not support port forwarding")