Merge pull request #1758 from dgquintas/iomgr_managed_cbs

src/core/channel/child_channel.c

src/core/iomgr/fd_posix.c

src/core/iomgr/fd_posix.h

src/core/iomgr/iomgr.c

src/core/iomgr/iomgr.h

src/core/iomgr/iomgr_internal.h

src/core/iomgr/pollset_posix.c

src/core/iomgr/pollset_windows.c

src/core/iomgr/socket_windows.c

src/core/iomgr/socket_windows.h

src/core/iomgr/tcp_posix.c

src/core/security/credentials.c

src/core/surface/call.c

src/core/surface/channel.c

src/core/surface/server.c

test/core/iomgr/alarm_test.c