Milo: Use custom config caching layer

luci_config/server/cfgclient/config_test.go

Index: luci_config/server/cfgclient/config_test.go
diff --git a/luci_config/server/cfgclient/config_test.go b/luci_config/server/cfgclient/config_test.go
index 7607f4006b0713829b968828d3a276179fbe4df2..71ae7776e7dda180cb788b7e4c693d3ca437945a 100644
--- a/luci_config/server/cfgclient/config_test.go
+++ b/luci_config/server/cfgclient/config_test.go
@@ -9,6 +9,7 @@ import (
 	"net/url"
 	"testing"
 
+	"github.com/luci/luci-go/common/config"
 	"github.com/luci/luci-go/common/errors"
 	"github.com/luci/luci-go/luci_config/server/cfgclient/backend"
 
@@ -54,6 +55,10 @@ func (tb *testingBackend) ConfigSetURL(c context.Context, configSet string, p ba
 	return tb.url, tb.err
 }
 
+func (tb *testingBackend) GetConfigInterface(c context.Context, a backend.Authority) config.Interface {
+	panic("not supported")
+}
+
 func (tb *testingBackend) cloneItems() []*backend.Item {
 	clones := make([]*backend.Item, len(tb.items))
 	for i, it := range tb.items {