…of state

…ble with caches

Using a role generic was incompatible with the type erasure
of the introspection cache.
Judging from the discussion in smartive#550
the original intention was to introduce a convenience feature, for library users.
However, the same behaviour can be easily achieved by a simple extention trait,
and does not need to be support by this crate itself to achieve the same convenience.
As a result, the generic role type parameter is removed.

* don't lock on every request
* use a ttl expiry for better token security

…onse