Merge master into let-invalidate-channels

Had to manually resolve import conflicts in InteropTests.m
To be fair, the merge algorithm should have been able to do it.

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+# gRPC command line tool
+
+## Overview
+
+This document describes the command line tool that comes with gRPC repository. It is desireable to have command line
+tools written in other languages to roughly follow the same syntax and flags.
+
+At this point, the tool needs to be built from source, and it should be moved out to grpc-tools repository as a stand
+alone application once it is mature enough.
+
+## Core functionality
+
+The command line tool can do the following things:
+
+- Send unary rpc.
+- Attach metadata and display received metadata.
+- Handle common authentication to server.
+- Find the request/response types from a given proto file.
+- Read proto request in text form.
+- Read request in wire form (for protobuf messages, this means serialized binary form).
+- Display proto response in text form.
+- Write response in wire form to a file.
+
+The command line tool should support the following things:
+
+- List server services and methods through server reflection.
+- Infer request/response types from server reflection result.
+- Fine-grained auth control (such as, use this oauth token to talk to the server).
+- Send streaming rpc.
+
+## Code location
+
+To use the tool, you need to get the grpc repository and in the grpc directory execute
+
+```
+$ make grpc_cli
+```
+
+The main file can be found at
+https://github.com/grpc/grpc/blob/master/test/cpp/util/grpc_cli.cc
+
+## Usage
+
+### Basic usage
+
+Send a rpc to a helloworld server at `localhost:50051`:
+
+```
+$ bins/opt/grpc_cli call localhost:50051 SayHello examples/protos/helloworld.proto \
+    "name: 'world'"  --enable_ssl=false
+```
+
+On success, the tool will print out
+
+```
+Rpc succeeded with OK status
+Response: 
+ message: "Hello world"
+```
+
+The `localhost:50051` part indicates the server you are connecting to. `SayHello` is (part of) the
+gRPC method string. Then there is the path to the proto file containing the service definition,
+if it is not under current directory, you can use `--proto_path` to specify a new search root.
+`"name: 'world'"` is the text format of the request proto message. 
+We are not using ssl here by `--enable_ssl=false`. For information on more
+flags, look at the comments of `grpc_cli.cc`.
+
+### Send non-proto rpc
+
+For using gRPC with protocols other than probobuf, you will need the exact method name string
+and a file containing the raw bytes to be sent on the wire
+
+```
+$ bins/opt/grpc_cli call localhost:50051 /helloworld.Greeter/SayHello --input_binary_file=input.bin \
+    --output_binary_file=output.bin
+```
+On success, you will need to read or decode the response from the `output.bin` file.