etcdserver, clientv3: handle a case of expired auth token

This commit adds a mechanism of handling a case of expired auth token
to clientv3. If a server returns an error code
grpc.codes.Unauthenticated, newRetryWrapper() tries to get a new token
and use it as an option of PerRPCCredential.

Fixes https://github.com/coreos/etcd/issues/7012

etcdserver/api/v3rpc/util.go

@@ -93,6 +93,8 @@ func togRPCError(err error) error {
 		return rpctypes.ErrGRPCPermissionNotGranted
 	case auth.ErrAuthNotEnabled:
 		return rpctypes.ErrGRPCAuthNotEnabled
+	case etcdserver.ErrInvalidAuthToken:
+		return rpctypes.ErrGRPCInvalidAuthToken
 	default:
 		return grpc.Errorf(codes.Unknown, err.Error())
 	}