Change the expiration behavior of sessionToken

See https://bugzilla.mozilla.org/show_bug.cgi?id=1022678

 - [x] Automatically prune ONE_DAY old not validated sessionToken
 - [x] cleanSession should remove both redis and longStorage keys.
 - [x] As soon as a sessionToken is validated store it in a long live storage
 - [x] Store the last usage and the date of activation of a sessionToken
 - [x] As soon as the persistentStorage is set prune volatile data.

We will use redis for the validation flow and as soon as the MSISDN is validated, drop validation data and build a longlive database entry to store MSISDN, creation date and last update date
You can have persistent storage using redis or volatile storage using mysql or dynamodb.

The backend and it nature are two different thing.
By default you will use the same storage for everything, except for certain cases where you will want to store validation data (volatile) in one store and validated data (persistent) in another.

At first I did what you said, but then @ametaireau convince me that it is better if we don't have to bother the user with this volatile/persistent issue if he doesn't need it.