Add document check job queue, abstract out lsp-editor%

[6cdh]

I did

• remove all match on struct Doc, Use struct accessor instead. So doc can contain extra hidden fields.
• add a extra middle layer doc.rkt between check-syntax, doc-trace and text-document, and move some code from text-document.rkt to doc.rkt. The Doc-text, Doc-trace should not be called in text-document.rkt, it should call a proxy method in doc.rkt instead.
• isolate check-syntax so that it's not called here and there. It's only called in doc-run-check-syntax now.
• clear check-syntax. It no longer sends diagnostics directly, instead, it returns diagnostics.
• use a unified editor class lsp-editor% instead of using text% or racket:text% everywhere. It would be useful if we write our own implementation to solve the gui problem.
• add a 90 seconds timeout for expand, should fix #109.
• improve the queue-only-latest.rkt implementation.
  • in the old design, there is at most a running check-syntax job and a pending job in the whole program. Any new incoming job will replace the pending job.
  • in the new design in scheduler.rkt, every document that is identified by uri can have a running job. Any new incoming job for this document will kill the old running job and run itself.
• make the initial trace object to be an empty build-trace% object not #f. It will fix many crash problem if check-syntax never succeed.
• other small improvements

The following issues will be fixed

• 101

• 97

[dannypsnl]

I think the title can be "Add document check job queue, abstract out lsp-editor%"

[6cdh]

The new fix editor crash commit fixed a random editor crash with this message:

sequence-contract-violation: negative: method insert cannot be called

It's a well known bug that no one knows how it happened. https://github.com/racket/drracket/issues/522 I found it today and added a workaround that recreate a new text% to replace to old one.

[6cdh]

Everything should work fine right now to be merged.