[wip] defer initialization until event loop is available

[bollwyvl]

References

• minimal fix for Slow startup on a cloud-hosted Hub #851

Code changes

• moves full initialization off critical path to starting JupyterServerApp
  • when it does initialize, moves potentially-expensive actions to a thread pool executor to not inadvertently block other early functions
  • when done, set a ready flag
• makes more of the handler methods asynchronous, and wait on the ready flag

User-facing changes

• slow startup would now be deferred, allowing the server to start more quickly
• the initial request for language status might be delayed some
  • typically it takes a while and many hundreds of HTTP requests, to get all the way up to a full-rendered document that could even request whether a language server is available
• the debug log message comes after the server announces its location, making it harder to find the server URL

Backwards-incompatible changes

• n/a, though some of the internal methods

Future Work

• this is still "blocks" (from an information theory perspective) the ability to start any session until all servers are discovered
  • theoretically with an isComplete flag on the response, we could handle partial setup better, and hoist more errors to the client log

Chores

• linted
• tested
  • the new server initialize code isn't under unit test, probably should be
  • benchmark startup somehow?
• documented
• changelog entry

[github-actions]

👈 Launch a binder notebook on branch bollwyvl/jupyterlab-lsp/gh-851-lazy-initialization

[bollwyvl]

Continued in #882