Change the expiration behavior of sessionToken

See https://bugzilla.mozilla.org/show_bug.cgi?id=1022678

 - [ ] Store the last usage and the date of activation of a sessionToken
 - [x] Automatically prune ONE_DAY old not validated sessionToken
 - [ ] As soon as a sessionToken is validated store it in a long live storage
 - [ ] cleanSession should remove both redis and longStorage keys.

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
I don't understand the comment here.