Merge pull request #10194 from ctiller/atomic-timers

doc/environment_variables.md

include/grpc/impl/codegen/atm_gcc_atomic.h

include/grpc/impl/codegen/atm_gcc_sync.h

include/grpc/impl/codegen/atm_windows.h

include/grpc/impl/codegen/port_platform.h

include/grpc/support/tls.h

src/core/lib/iomgr/ev_epoll_linux.c

src/core/lib/iomgr/ev_poll_posix.c

src/core/lib/iomgr/timer.h

src/core/lib/iomgr/timer_generic.c

src/core/lib/iomgr/timer_generic.h

src/core/lib/iomgr/timer_heap.c

test/core/end2end/tests/max_connection_age.c

test/core/end2end/tests/max_connection_idle.c

test/core/iomgr/timer_heap_test.c

test/core/iomgr/timer_list_test.c