WIP: Add deploy without creating a state file

[adisbladis]

Create an in-memory SQLite db if --no-state is passed to deploy.
This is probably the easiest way possible for us to add support for deployments without using a state file.

If --no-state is passed a create is implicitly done in the same process.

While this works perfectly fine with the none backend it's marked WIP because it needs docs.

Based on top of #1240 since they touch a lot of the same code & meant to be used together with #1247.

[cleverca22]

main thing i can see being lost with this change, is that the stateVersion is normally queried on the first deploy and held in the state file, to deploy compatible nixos's in the future

but the user must now manage stateVersion themselves, in the deployment files

nixops rollbacks also require the uuid of the deployment to remain constant, so it can find the generations

[grahamc]

Overall, I like this a lot.

I think --no-state should not be an option. Instead, I think it should be a property of the network.nix itself. For example, a user who creates a network which spawns a bunch of wildly expensive boxes, but passes --no-state... and then NixOps fires-and-forgets a big bill for them.

In cases like this, I think the NixOps should refuse to do things like create resources when state is disabled. Thus, always be applied (or not applied), thus part of the network definition.

[grahamc]

I just force-pushed a rebase on to master.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/tweag-nix-dev-update/6525/1

[adisbladis]

#1264 which builds on this is a better and more comprehensive solution. Closing.