Stripping out master channel as a concept

src/core/channel/channel_stack.c

src/core/channel/channel_stack.h

src/core/channel/client_channel.c

src/core/channel/client_channel.h

src/core/channel/client_uchannel.c

src/core/channel/client_uchannel.h

src/core/channel/connected_channel.c

src/core/channel/subchannel_call_holder.c

src/core/channel/subchannel_call_holder.h

src/core/client_config/subchannel.c

src/core/client_config/subchannel.h

src/core/surface/channel.c

src/core/surface/secure_channel_create.c

test/core/end2end/fixtures/h2_uchannel.c