Merge github.com:grpc/grpc into minimal

doc/environment_variables.md

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/port_platform.h

include/grpc/support/tls.h

src/core/ext/census/grpc_filter.c

src/core/ext/filters/client_channel/client_channel.c

src/core/ext/filters/client_channel/subchannel.c

src/core/ext/filters/client_channel/subchannel.h

src/core/ext/filters/load_reporting/load_reporting_filter.c

src/core/ext/transport/chttp2/transport/chttp2_transport.c

src/core/ext/transport/cronet/transport/cronet_transport.c

src/core/lib/channel/channel_stack.c

src/core/lib/channel/channel_stack.h

src/core/lib/channel/compress_filter.c

src/core/lib/channel/connected_channel.c

src/core/lib/channel/deadline_filter.c

src/core/lib/channel/deadline_filter.h

src/core/lib/channel/http_client_filter.c