[Auth] Potential race condition acquiring credentials via 'AuthProvider' interface and 'cbauth'

What's the issue?
I believe there's a potential race condition when acquiring credentials using the 'AuthProvider' interface, the general steps would be as follows (when applied to 'cbbackupmgr'):

1. SDK prepares to dispatch request
2. SDK asks 'cbbackupmgr' the credentials for a given host:port
3. 'cbbackupmgr' uses gRPC to ask for credentials from 'cbbs' (the backup service) via 'cbauth'
4. Cluster node required for step 1 is removed from the cluster
5. 'cbbs' fails to find credentials for the given host:port
6. 'cbbackupmgr' falls back to the credentials provided via the CLI (which will be wrong due to being a system user)

What's the fix?
'cbbackupmgr' should be able to inform the SDK that we can't provide credentials for the given host:port and that it should retry the request (preferably after a cluster config update) so that it's dispatched to a different node (or fails because no other nodes run the desired service).