Introduced a registry for LB policies.

BUILD

Makefile

build.yaml

gRPC.podspec

src/core/client_config/lb_policies/pick_first.c

src/core/client_config/lb_policies/pick_first.h

src/core/client_config/lb_policy_factory.c

src/core/client_config/lb_policy_factory.h

src/core/client_config/lb_policy_registry.c

src/core/client_config/lb_policy_registry.h

src/core/client_config/resolvers/dns_resolver.c

src/core/client_config/resolvers/sockaddr_resolver.c

src/core/client_config/resolvers/zookeeper_resolver.c

src/core/surface/init.c

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