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
> proxy is there to be able to use two differents backend one for volatileStorage and one for persistentStorage.

I don't understand what prevents us from using the same engine for both storages without having to define a 'proxy' thing in the configuration on the top of both. e.g.:

```json
{
"storageA": {"engine": "redis", ...},
"storageB": {"engine": "redis", ...},
}   
```

>  proxy is there just if you want to mix two backend for the same storage.

this confuses me. does proxy offer specific things ?