Add Persistent Storage Support to V2Ray

[xiaokangwang]

This merge request add persistent storage support for V2Ray. This would allow client and server to save state at provided location. Currently this is implemented using plain file.

To add a file system based storage, add the following to the service section of the config:

    "filesystemstorage": {
      "state_storage_root": "WorkDir",
      "instance_name": "inst0",
      "protojson": false
    }

Currently backgroundObservatory supports save observation result so that the remote server status would outlive server restart.