Merge branch 'hide-the-pollset' into cleaner-posix2

src/core/iomgr/ev_poll_and_epoll_posix.c

src/core/iomgr/iomgr.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_posix.c

src/core/iomgr/pollset_set_posix.h

src/core/iomgr/pollset_windows.c

src/core/iomgr/pollset_windows.h

src/core/iomgr/tcp_posix.c

src/core/iomgr/timer.h

src/core/iomgr/workqueue_posix.h

src/core/security/google_default_credentials.c

src/core/surface/completion_queue.c

test/core/client_config/set_initial_connect_string_test.c

test/core/end2end/fixtures/h2_full+poll+pipe.c

test/core/end2end/fixtures/h2_full+poll.c

test/core/end2end/fixtures/h2_ssl+poll.c