Merge pull request #5433 from ctiller/hide-the-pollset-set-redux

src/core/channel/client_channel.c

src/core/client_config/lb_policies/pick_first.c

src/core/client_config/lb_policies/round_robin.c

src/core/client_config/lb_policy.c

src/core/client_config/lb_policy.h

src/core/client_config/subchannel.c

src/core/httpcli/httpcli.c

src/core/httpcli/httpcli.h

src/core/iomgr/fd_posix.c

src/core/iomgr/pollset.h

src/core/iomgr/pollset_multipoller_with_epoll.c

src/core/iomgr/pollset_multipoller_with_poll_posix.c

src/core/iomgr/pollset_posix.c

src/core/iomgr/pollset_posix.h

src/core/iomgr/pollset_set.h

src/core/iomgr/pollset_set_posix.c

src/core/iomgr/pollset_set_posix.h

src/core/iomgr/pollset_set_windows.c

src/core/iomgr/pollset_set_windows.h

src/core/iomgr/pollset_windows.c