Merge branch 'master' into node_protobuf_js_6_upgrade

BUILD

CMakeLists.txt

Makefile

binding.gyp

build.yaml

config.m4

doc/environment_variables.md

gRPC-Core.podspec

grpc.gemspec

include/grpc/impl/codegen/atm_gcc_atomic.h

include/grpc/impl/codegen/atm_gcc_sync.h

include/grpc/impl/codegen/atm_windows.h

include/grpc/impl/codegen/grpc_types.h

include/grpc/impl/codegen/port_platform.h

include/grpc/support/tls.h

package.xml

src/compiler/cpp_generator.cc

src/compiler/cpp_generator.h

src/compiler/cpp_generator_helpers.h

src/compiler/cpp_plugin.cc