Long-lived tokens

[graft]

Janus currently expires tokens after 24 hours. This is required because the token, once issued, cannot be invalidated except by a recycling of secrets (that is, the Janus public key) across all Etna applications.

However, in many instances, mostly long-lived jobs that might take several days to execute, the short-lived token is a nuisance. There are several ways to avoid this nuisance, mostly the /refresh_token and /generate endpoints, which either let you keep a current token alive before it expires, or sign for a new token using a registered public key. Both of these methods involve contacting Janus to serially update the token.

In these instances we'd probably prefer a single, long-lived token that works the same as the regular Janus token, but doesn't require constant refreshing. However, as noted above, we want to be able to invalidate the long-lived token somehow.

To implement, we add a /long_token endpoint that allows the issuing of long-lived tokens. This token is modified in a few ways:

1. Only one project/role is allowed in its payload.
2. It has a verify: true bit in its payload.
3. The expiration is some suitably long period, perhaps 60 days.

The usage is identical; Etna::Auth, in the process of verifying the token (i.e., for each transaction to any Etna app with this token), notes the presence of verify: true and phones Janus at /validate_token, which confirms that the credentials in the token are still valid according to Janus's db. Etna::Auth then rejects or approves on the basis of this received confirmation.