It fixes a bug in which ImageDecoder and RequestServer
do not exit because their connections don't close.
This makes the shutdown behavior match the Linux version,
which receives FD_READ | FD_HANGUP on socket close, and
TransportSocket::read_as_much_as_possible_without_blocking calls
schedule_shutdown when read from a socket returns 0 bytes.
On Windows, we have to explicitly call WIN32 shutdown to receive
notification FD_CLOSE.
