v2.3.1

Release Notes for the New Version:

• Support a verifiable payment mechanism;
• Support mongo atlas.
• Support the CORS option on the configure file.
• Support update or delete one on the v2 database APIs.
• Add documents for the v2 APIs.
• Add the v2 about APIs.
• Refine the v2 backup APIs.
• Fix bug for counting storage usage about the v1 files API.
• Test cases improvement.