implement support for structured replies in both nbd-client and nbd-server

[nhooyr]

Neither support structured replies (nbd-client doesn't support sending the option to negotiate structured replies)

[yoe]

Yes, I know it hasn't been implemented yet. This will happen when I find the time, at least for the server side.

For the client side, kernel support will be needed.

[nhooyr]

Going to close as it has been a long time and I'm not sure if this is relevant anymore.

[yoe]

It's definitely still relevant. I've been looking at working on this, too.

[yoe]

Server side has been implemented as of 3.25.

[nhooyr]

This is my oldest filed issue on which there's been meaningful progress. Good stuff @yoe 🍻