Update federation target to etcd/clientv3 (#6119)

With CoreDNS now supporting etcdv3 as the DNS backend, we
can update our federation target to etcdv3. Users will now be
able to use etcdv3 server as the federation backbone.

Minio will update bucket data to etcdv3 and CoreDNS can pick
that data up and serve it as bucket style DNS path.

vendor/google.golang.org/grpc/interceptor.go

@@ -1,33 +1,18 @@
 /*
  *
- * Copyright 2016, Google Inc.
- * All rights reserved.
+ * Copyright 2016 gRPC authors.
  *
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted provided that the following conditions are
- * met:
+ * 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
  *
- *     * Redistributions of source code must retain the above copyright
- * notice, this list of conditions and the following disclaimer.
- *     * Redistributions in binary form must reproduce the above
- * copyright notice, this list of conditions and the following disclaimer
- * in the documentation and/or other materials provided with the
- * distribution.
- *     * Neither the name of Google Inc. nor the names of its
- * contributors may be used to endorse or promote products derived from
- * this software without specific prior written permission.
+ *     http://www.apache.org/licenses/LICENSE-2.0
  *
- * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
- * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
- * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
- * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
- * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
- * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
- * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
- * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
- * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
- * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
- * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ * 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.
  *
  */
 
@@ -42,15 +27,15 @@ type UnaryInvoker func(ctx context.Context, method string, req, reply interface{
 
 // UnaryClientInterceptor intercepts the execution of a unary RPC on the client. invoker is the handler to complete the RPC
 // and it is the responsibility of the interceptor to call it.
-// This is the EXPERIMENTAL API.
+// This is an EXPERIMENTAL API.
 type UnaryClientInterceptor func(ctx context.Context, method string, req, reply interface{}, cc *ClientConn, invoker UnaryInvoker, opts ...CallOption) error
 
 // Streamer is called by StreamClientInterceptor to create a ClientStream.
 type Streamer func(ctx context.Context, desc *StreamDesc, cc *ClientConn, method string, opts ...CallOption) (ClientStream, error)
 
 // StreamClientInterceptor intercepts the creation of ClientStream. It may return a custom ClientStream to intercept all I/O
-// operations. streamer is the handlder to create a ClientStream and it is the responsibility of the interceptor to call it.
-// This is the EXPERIMENTAL API.
+// operations. streamer is the handler to create a ClientStream and it is the responsibility of the interceptor to call it.
+// This is an EXPERIMENTAL API.
 type StreamClientInterceptor func(ctx context.Context, desc *StreamDesc, cc *ClientConn, method string, streamer Streamer, opts ...CallOption) (ClientStream, error)
 
 // UnaryServerInfo consists of various information about a unary RPC on
@@ -63,7 +48,9 @@ type UnaryServerInfo struct {
 }
 
 // UnaryHandler defines the handler invoked by UnaryServerInterceptor to complete the normal
-// execution of a unary RPC.
+// execution of a unary RPC. If a UnaryHandler returns an error, it should be produced by the
+// status package, or else gRPC will use codes.Unknown as the status code and err.Error() as
+// the status message of the RPC.
 type UnaryHandler func(ctx context.Context, req interface{}) (interface{}, error)
 
 // UnaryServerInterceptor provides a hook to intercept the execution of a unary RPC on the server. info