Slow persistence code checking

[ahead4ltd]

Currently persistence codes are stored in json format on the users table. When checking for a persistence code it has to perform (on average) a half table row scan just to determine the persistence code. As you can imagine, with a large users table this is a considerable performance hit. Ideally, the persistence codes should be split into a separate pivot table like so:

 user_id      persistant_code [indexed]

[brunogaspar]

This has changed, you can check the feature/updates branch if you want.