Implement a server-side config service interface.

luci_config/server/cfgclient/backend/doc.go

Index: luci_config/server/cfgclient/backend/doc.go
diff --git a/luci_config/server/cfgclient/backend/doc.go b/luci_config/server/cfgclient/backend/doc.go
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+// 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 implements configuration client backend interface and
+// associated data types.
+//
+// The backend package implements a generalized implementation-oriented
+// interface to configuration services that can be layered. Various backend
+// implementations are best defined out of this package.
+//
+// All non-user-facing operations of cfgclient will operate on backend types.
+package backend