Move cluster config from raft to API.

Fixes #853. Close #854. Previously, there was an unprotected endpoint in
raft to return the cluster config that would include user hashes. This
endpoint is useful for debugging purposes so I restructured it and moved
it to the API. It ensures the requesting user is a cluster admin.

Cluster config will now return all of the cluster state including
servers, CQs, shards, etc.