Custom serializers for ChangeKindCache, including protobuf

Change-Id: I7f2e56f78fd346170922c366c57465d6dfb136ef

proto/cache.proto

@@ -0,0 +1,27 @@
+// Copyright (C) 2018 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto3";
+
+package gerrit.cache;
+
+option java_package = "com.google.gerrit.server.cache.proto";
+
+// Serialized form of com.google.gerrit.server.change.CHangeKindCacheImpl.Key.
+// Next ID: 4
+message ChangeKindKeyProto {
+  bytes prior = 1;
+  bytes next = 2;
+  string strategy_name = 3;
+}