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.

cli/daemon/daemon.go

@@ -19,7 +19,7 @@ func InitCmd(cmd *cobra.Command) error {
 	}
 
 	daemonCmd.Flags().Bool("compression", true, "Compress measured values")
-	daemonCmd.Flags().Uint("cached-values", 500, "Number of cached values before saveing into the storage endpoint")
+	daemonCmd.Flags().Uint("cached-values", 500, "Number of cached values before saveing into the backend")
 	daemonCmd.Flags().Float64("round", 0.5, "Round values. The value 0 deactivates the function")
 	cmd.AddCommand(daemonCmd)