Part of #267: Implement in Agora version upgrader

[Geod24]

In order to fix the issue, we need two components:
- The ability to update the node's persistent storage to accomodate for format / schema changes;
- The ability to automatically download and install a new update whenever available;

This implements the former functionality. The later will be handled outside of Agora.

This still need:

• Proper testing
• A fix to semver: https://github.com/dcarp/semver/pull/5

Submitting in case anyone want to give feedback on the design.