Merge github.com:grpc/grpc into optionalize_client_config

include/grpc++/impl/codegen/async_stream.h

include/grpc++/impl/codegen/async_unary_call.h

include/grpc++/impl/codegen/call.h

include/grpc++/impl/codegen/client_context.h

include/grpc++/impl/codegen/client_unary_call.h

include/grpc++/impl/codegen/method_handler_impl.h

include/grpc++/impl/codegen/server_context.h

include/grpc++/impl/codegen/sync_stream.h

include/grpc/impl/codegen/grpc_types.h

src/core/ext/client_config/client_channel.c

src/core/ext/client_config/lb_policy.c

src/core/ext/client_config/lb_policy.h

src/core/ext/client_config/subchannel.c

src/core/ext/lb_policy/pick_first/pick_first.c

src/core/ext/lb_policy/round_robin/round_robin.c

src/core/lib/channel/http_client_filter.c

src/core/lib/channel/subchannel_call_holder.c

src/core/lib/channel/subchannel_call_holder.h

src/core/lib/surface/call.c

src/core/lib/transport/transport.h