Chunk server: implement access control with chunk access tokens that have write id and lease id subjects bit set. This is needed to allow write master to issue chunk access tokens bound to both chunk and write id. Such token can have practically unlimited life time, as write id is seeded using random generation, and therefore chunk / write id [and chunk server's current key] combination is unique for all practical purposes. The write id lifetime, in turn, is bounded by the write lease, and chunk server's key lifetime.