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
> Mixing means use a backend for volatile storage methods and another one for persistent storage methods.

It is already the case when you write 

```json
{
"storageA": {"engine": "redis", ...},
"storageB": {"engine": "redis", ...},
} 
```

> I want to use a storage to store data.

Yes sure. You did not answer to those questions:

* Is there any added feature than simply pointing the same backend for both storages ? 
* If not, what is the added value besides verbosity in the config ?