Merge pull request #3467 from ctiller/can-we-be-single-threaded

src/core/iomgr/exec_ctx.c

src/core/iomgr/exec_ctx.h

src/core/iomgr/iocp_windows.c

src/core/iomgr/iocp_windows.h

src/core/iomgr/iomgr.c

src/core/iomgr/iomgr_internal.h

src/core/iomgr/iomgr_posix.c

src/core/iomgr/iomgr_posix.h

src/core/iomgr/iomgr_windows.c

src/core/iomgr/pollset_windows.c

src/core/iomgr/pollset_windows.h

...Grpc.Core.Tests/Internal/CompletionQueueSafeHandleTest.cs