Merge github.com:grpc/grpc into call_cases

BUILD

CMakeLists.txt

Makefile

binding.gyp

build.yaml

config.m4

doc/combiner-explainer.md

gRPC-Core.podspec

grpc.gemspec

include/grpc++/support/channel_arguments.h

include/grpc/impl/codegen/atm.h

package.xml

setup.py

src/core/ext/census/grpc_filter.c

src/core/ext/client_channel/client_channel.c

src/core/ext/client_channel/client_channel_plugin.c

src/core/ext/client_channel/proxy_mapper_registry.c

src/core/ext/client_channel/retry_throttle.c

src/core/ext/client_channel/retry_throttle.h

src/core/ext/client_channel/subchannel.c