R1:fa7bb8ff7a50

Add `cluster.addresses` and require membership before accepting cluster authentication tokens

Summary:
Ref T2783. Ref T6706.

- Add `cluster.addresses`. This is a whitelist of CIDR blocks which define cluster hosts.
- When we recieve a request that has a cluster-based authentication token, require the cluster to be configured and require the remote address to be a cluster member before we accept it.
- This provides a general layer of security for these mechanisms.
- In particular, it means…