Implement config service cache on top of datastore

appengine/gaeconfig/cache.go

 // Copyright 2015 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 gaeconfig
 
 import (
         "encoding/hex"
         "time"
 
@@ skipping 63 matching lines @@
                                                 log.ErrorKey: err,
                                                 "key":        k,
                                         }.Warningf(c, "Failed to cache config.")
                                 }
 
                                 // Return the loaded value.
                                 return v, nil
 
                         default:
                                 // Unknown memcache error.
-                                log.Fields{
-                                        log.ErrorKey: err,
-                                        "key":        k,
-                                }.Warningf(c, "Failed to decode memcached config.")
-                                return l(c, key, nil)
+                                return nil, errors.Annotate(err).Reason("failed to load memcache config").Err()
                         }
                 },
         }
 }
 
 func memcacheKey(key *caching.Key) string { return hex.EncodeToString(key.ParamHash()) }