nixd/fork: forking concurrency

[inclyc]

I'd like to eval workspace files in a separated process. When we update the workspace, we call fork() to lazily duplicate nixd itself. And, the server turns itself into Worker mode, which means:

1. Enable language feature by calling nix::init*, and also start the GC.
2. Do not response workspace update request, just fixing the workspace, with evaluation runs at once.
3. Handle LSP features, like Hover
4. The Worker itself runs in single-thread mode.

The main process it self redirects LSP requests to versioned workers, and just choose the best result, and thus should be cancellable.

The workers are managed in a deque, which means we can kill outdated processes (and thus free the memory).

regression status:

• diagnostic
• completion
• hover

[inclyc]

Now all features are implemented in forking architecture, however looks like something is not working in github ci, but I cannot reproduce locally.