Sanitize unsubscription to be callback preserving

src/core/channel/client_channel.c

src/core/channel/client_uchannel.c

src/core/client_config/lb_policies/pick_first.c

src/core/client_config/lb_policy.c

src/core/client_config/lb_policy.h

src/core/client_config/subchannel.c

src/core/client_config/subchannel.h

src/core/iomgr/fd_posix.h

src/core/iomgr/iomgr.c

src/core/transport/chttp2_transport.c

src/core/transport/connectivity_state.c

src/core/transport/connectivity_state.h

src/core/transport/transport.h