Separate timer checking from pollsets

BUILD

build.yaml

gRPC.podspec

grpc.gemspec

package.json

src/core/iomgr/iocp_windows.c

src/core/iomgr/iomgr.c

src/core/iomgr/pollset_posix.c

src/core/iomgr/pollset_windows.c

src/core/iomgr/timer.c

src/core/iomgr/timer.h

src/core/iomgr/timer_internal.h

src/core/surface/completion_queue.c

test/core/iomgr/timer_list_test.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