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
> store validation data (volatile) in one store and validated data (persistent) in another.

ah! that's where the confusion is for me.

I think we have two storages. one that stores **validation** data and one **validated** data, right ?  here you are making the assumption that the validation data is volatile and the validated data is not. But in the same time you said earlier that they both can use a volatile or persistent backend...

> 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.

Again this is very confusing. You are talking about "volatile/persistent issue" when you are describing the 2 storages.

This below is very confusing... a persistent storage configuration using redis... : 

```json
"persistentStorageConf": {
       "engine": "redis"
}
```

Where I think it could be:

```json
"validationStorageConf": {
       "engine": "redis"
}
```