adapter, integration: split out grpc adapters from grpcproxy package

Break cyclic dependency:
clientv3/naming <-> integration <-> v3client <-> grpcproxy <-> clientv3/naming

proxy/grpcproxy/adapter/doc.go

@@ -0,0 +1,17 @@
+// Copyright 2017 The etcd Authors
+//
+// 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.
+
+// Package adapter provides gRPC adapters between client and server
+// gRPC interfaces without needing to go through a gRPC connection.
+package adapter