Further client config work

src/core/channel/client_channel.c

src/core/client_config/client_config.c

src/core/client_config/lb_policies/pick_first.c

src/core/client_config/lb_policy.h

src/core/client_config/resolver_factory.c

src/core/client_config/resolver_registry.c

src/core/client_config/resolver_registry.h

src/core/client_config/resolvers/dns_resolver.c

src/core/client_config/resolvers/dns_resolver.h

src/core/client_config/subchannel.c

src/core/client_config/subchannel.h

src/core/client_config/subchannel_factory.c

src/core/client_config/uri_parser.c

src/core/client_config/uri_parser.h

src/core/surface/init.c

test/core/end2end/fixtures/chttp2_fullstack.c