Merge pull request #9696 from yang-g/network_status_shard

include/grpc/support/useful.h

src/core/lib/iomgr/network_status_tracker.c

src/core/lib/iomgr/timer_generic.c

src/core/lib/support/cpu_posix.c

test/core/end2end/tests/network_status_change.c