fix: renamed storage endpoint into dsn

Changes:
- Renamed storage endpoint into dsn (data source name).
- Add additional dsn fallback property. This dsn will be used in futes
  to store informations, if the main dsn backend does not work
  correctly. For example, if no connection can be established over the
  network to a database.

pkg/config/config.go

@@ -20,9 +20,10 @@ var (
 
 // Config represent the configuration
 type Config struct {
-	Device          *types.Device   `json:"device"`
-	Sensors         []*types.Sensor `json:"sensors"`
-	StorageEndpoint string          `json:"storage_endpoint"`
+	Device      *types.Device   `json:"device"`
+	Sensors     []*types.Sensor `json:"sensors"`
+	DSN         string          `json:"dsn"`
+	DSNFallback string          `json:"dsn_fallback"`
 }
 
 // AddSensor add a new sensor