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

tokenserver/appengine/impl/certconfig/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
 
 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
+                // Warmup ID -> CN mapping. Ignore result (it may be "no such CA"), this is
+                // fine, the mapping will be preloaded.
+                GetCAByUniqueID(c, 0)
                 return nil
         })
 }