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
this is quite confusing, because we're mixing here two things: the goal of the storage (what it stores) and its nature (volatile/persistent)

I think we should remove that confusion by removing the volatile/persistent names and define a specific name for each storage that gives a hint in what it stores.

Then the name engine can be redis or mysql or dynamodb - that informs us if its volatile or persistent.