Implement persistent CSR authorization database [service]

[allingeek]

Currently the Pollendina service uses an in memory database (golang map) to store token=>CN authorizations. In order to support horizontal scaling of the Pollendina service and durability, some persistent or distributed database should be adopted.

A backend plugin infrastructure would be nice. That way adopters could use Consul, etcd, Redis, etc.

Alternatively, clustering could be embedded into the service for service-to-service memory sharing. One reason this may be preferred is that this is access control data. Securing the storage of the authorization content is at least as important as providing the authorization mechanism in the first place.