Remove ChannelInterface and replace it with Channel

BUILD

Makefile

build.json

examples/pubsub/main.cc

examples/pubsub/publisher.cc

examples/pubsub/publisher.h

examples/pubsub/publisher_test.cc

examples/pubsub/subscriber.cc

examples/pubsub/subscriber.h

examples/pubsub/subscriber_test.cc

include/grpc++/async_unary_call.h

include/grpc++/channel.h

include/grpc++/client_context.h

include/grpc++/completion_queue.h

include/grpc++/create_channel.h

include/grpc++/credentials.h

include/grpc++/generic_stub.h

include/grpc++/impl/client_unary_call.h

include/grpc++/impl/internal_stub.h

include/grpc++/stream.h