token-server: Warmup some local caches in /_ah/warmup.

tokenserver/appengine/impl/delegation/warmup.go

 // Copyright 2017 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
-package certconfig
+package delegation
 
 import (
         "golang.org/x/net/context"
 
         "github.com/luci/luci-go/server/warmup"
 )
 
 func init() {
-        warmup.Register("tokenserver/appengine/impl/certconfig", func(c context.Context) error {
-                // TODO
-                return nil
+        warmup.Register("tokenserver/appengine/impl/delegation", func(c context.Context) error {
+                _, err := DelegationConfigLoader(c)
+                return err
         })
 }