2.3.0a1759946524

CG-1904  improve expired session ux (#45)

*    Improve the user experience around old stale sessions that appear to be
    initialized, but are actually expired. This is done by providing the new
    utility method Auth.ensure_request_authenticator_is_ready().

*    Save computed expiration time and issued time in token files. This allows
    for the persistence of this information when dealing with opaque tokens.

*    Support non-expiring tokens.