Milo: Use custom config caching layer

luci_config/server/cfgclient/backend/backend.go

 // Copyright 2016 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 backend
 
 import (
         "net/url"
 
+        "github.com/luci/luci-go/common/config"
+
         "golang.org/x/net/context"
 )
 
 // FormatSpec is a specification for formatted data.
 type FormatSpec struct {
         // Formatter is the supported destination Resolver format for this item.
         // Backends (notably the FormatterBackend) may project the Item into this
         // format.
         //
         // An empty string means the original config service format.
@@ skipping 36 matching lines @@
         ServiceURL(context.Context) url.URL
 
         // Get retrieves a single configuration.
         Get(c context.Context, configSet, path string, p Params) (*Item, error)
 
         // GetAll retrieves all configurations of a given type.
         GetAll(c context.Context, t GetAllTarget, path string, p Params) ([]*Item, error)
 
         // ConfigSetURL returns the URL for the specified config set.
         ConfigSetURL(c context.Context, configSet string, p Params) (url.URL, error)
+
+        // GetConfigInterface returns the raw configuration interface of the backend.
+        GetConfigInterface(c context.Context, a Authority) config.Interface
 }
 
 // Factory is a function that generates a B given a Context.
 type Factory func(context.Context) B
 
 // configBackendKey is the Context key for the configuration backend.
 var configBackendKey = "github.com/luci/luci-go/server/config:backend"
 
 // WithBackend returns a derivative Context with the supplied Backend installed.
 func WithBackend(c context.Context, b B) context.Context {
         return WithFactory(c, func(context.Context) B { return b })
 }
 
 // WithFactory returns a derivative Context with the supplied BackendFactory
 // installed.
 func WithFactory(c context.Context, f Factory) context.Context {
         return context.WithValue(c, &configBackendKey, f)
 }
 
 // Get returns the Backend that is installed into the Context.
 //
 // If no Backend is installed in the Context, Get will panic.
 func Get(c context.Context) B {
         if f, ok := c.Value(&configBackendKey).(Factory); ok {
                 return f(c)
         }
         panic("no Backend factory is installed in the Context")
 }