Refresh remote file meta and file content.

[weilonge]

Dropbox provides an API to long-poll the file changes:
https://www.dropbox.com/developers-v1/core/docs#longpoll-delta

[weilonge]

For Sample JSON backend, the JSON file is loaded from File System for nodejs version. We need a file-watch feature to monitor a file, and chokidar is a choice to avoid using fs.watch.

[weilonge]

There are some action items have to be done here:

• file/meta change notification
• cursor has to be stored at launch and each update
• the detail of delta information
  • e.g. Dropbox API - delta
• the meta/data cache manager support to clear a specific path entry.

[weilonge]

Since #36 and #39 are landed, this is the last task should be done:

the meta/data cache manager support to clear a specific path entry.