Move subchannel_call_holder code into client_channel module.

BUILD

CMakeLists.txt

Makefile

binding.gyp

build.yaml

config.m4

gRPC-Core.podspec

grpc.gemspec

package.xml

src/core/ext/client_config/client_channel.c

src/core/ext/client_config/subchannel_call_holder.c

src/core/ext/client_config/subchannel_call_holder.h

src/python/grpcio/grpc_core_dependencies.py

tools/doxygen/Doxyfile.core.internal

tools/run_tests/sources_and_headers.json

vsprojects/vcxproj/grpc/grpc.vcxproj

vsprojects/vcxproj/grpc/grpc.vcxproj.filters

vsprojects/vcxproj/grpc_unsecure/grpc_unsecure.vcxproj

...jects/vcxproj/grpc_unsecure/grpc_unsecure.vcxproj.filters