Use parallel thread

[dannypsnl]

I will use this branch for a few days before I make PR ready.

[dannypsnl]

Emmm, obviously not as easy as I thought.

[6cdh]

Yes. The parallel threads generally give good boost. But it slows down by 2x in my test for expand (expand 10 racket files). It probably won't benefit from parallel because of its own complexity.

[shhyou]

If racket-langserver requires Racket >=9.0, I think it'd be better to name it racket-langserver2 or so.

[dannypsnl]

@shhyou Indeed, that's a problem.

I will close this PR, because the model, expansion is only going for the current editing file, where parallel thread can't help.

Also, I shall not force an upgrade, I think we can

1. wait until Racket users all use >=9.0
2. or create an experimental branch, creates a complete different interaction to documents