Tradeoff: poll or notify

Filler - need to demonstrate:

polling (server cannot tell client about async changes unless client polls):
client: NBD_CMD_RESIZE(0)
server: NBD_REPLY_TYPE_RESIZE response, including NBD_REPLY_DONE
do stuff
client: NBD_CMD_RESIZE(0)
server: NBD_REPLY_TYPE_RESIZE response, including NBD_REPLY_DONE

notify (client registers open-ended command to allow multiple async server responses as needed):
client: NBD_CMD_RESIZE(NBD_CMD_FLAG_NOTIFY)
do stuff
server: NBD_REPLY_TYPE_RESIZE response, excluding NBD_REPLY_DONE
do stuff
server: NBD_REPLY_TYPE_RESIZE response, excluding NBD_REPLY_DONE