cipd: Implement local cache for ResolveVersion(...) call with tags.

go/src/infra/tools/cipd/internal/messages/messages.proto

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+syntax = "proto2";
+
+package messages;
+
+// BlobWithSHA1 is a wrapper around a binary blob with SHA1 hash to verify
+// its integrity.
+message BlobWithSHA1 {
+  optional bytes blob = 1;
+  optional bytes sha1 = 2;
+}
+
+// TagCache stores a mapping (package name, tag) -> instance ID to speed up
+// subsequence ResolveVersion calls when tags are used.
+message TagCache {
+  message Entry {
+    optional string package = 1;
+    optional string tag = 2;
+    optional string instance_id = 3;
+  }
+
+  // Capped list of entries, most recently resolved is last.
+  repeated Entry entries = 1;
+}