Merge pull request #5950 from dgquintas/lb_policies_new_interface

src/core/lib/client_config/lb_policies/pick_first.c

src/core/lib/client_config/lb_policies/round_robin.c

src/core/lib/client_config/lb_policy_factory.c

src/core/lib/client_config/lb_policy_factory.h

src/core/lib/client_config/lb_policy_registry.c

src/core/lib/client_config/lb_policy_registry.h

src/core/lib/client_config/resolvers/dns_resolver.c

src/core/lib/client_config/resolvers/sockaddr_resolver.c

src/core/lib/client_config/resolvers/zookeeper_resolver.c

test/core/client_config/lb_policies_test.c