Merge github.com:grpc/grpc into version3

BUILD

CMakeLists.txt

Makefile

build.yaml

composer.json

src/compiler/php_generator.cc

src/compiler/php_generator.h

src/compiler/php_generator_helpers.h

src/compiler/php_plugin.cc

src/csharp/Grpc.Core.Tests/ClientServerTest.cs

src/csharp/Grpc.Core/Internal/BatchContextSafeHandle.cs

src/csharp/Grpc.Core/Internal/CallSafeHandle.cs

src/csharp/Grpc.Core/Internal/NativeMethods.cs

src/csharp/ext/grpc_csharp_ext.c

src/php/composer.json

src/php/tests/generated_code/AbstractGeneratedCodeTest.php

src/php/tests/generated_code/GeneratedCodeTest.php

...hp/tests/generated_code/GeneratedCodeWithCallbackTest.php

src/php/tests/interop/interop_client.php

src/python/grpcio/grpc/_channel.py