Entry JSON data update for when their controller has changed

[sveetch]

Since a Controller can change to add new fields, remove fields or edit field options, the saved JSON data in previous version won't fit anymore to the new Controller structure and options.

At first, the usage of entry JSON data must always be compatible with new Controller version such as it won't break the data listing.

Then we would need a safe way to upgrade data from previous controller version. For this we have added a "version" value on entry that is filled with current controller version, so we can more easily know what entries are up to date or not.

To start with this, we need to predict and choose how we manage these differences. Then we would be able to code the upgrade features. It should be at least available from a commandline since it is flawless for heavy migration when Controller have a lot of data. And maybe we could think of an Admin action, but it could be risky with huge process and request timeout.

• When a new field is added, the data must include it with a blank value (this would need a default value on definition);
• When an existing field is removed, the data must be cleaned to remove it;