Make background LSP requests wait for pending update

[bentsherman]

Close #23

When a delayed update is scheduled, "background" LSP requests like document link and document symbol will await that update, so that they don't receive stale information.

"Foreground" requests like hover hint, completion, goto definition, etc should either force-update or just query against the current state, so as to not keep the user waiting.

[bentsherman]

In vscode, the document link request seems to be delayed long enough to stay up to date, but the symbol request is not.

You can see this by showing the outline in the sidebar, then deleting a definition (e.g. function, process, workflow) in a script. The outline is updated "too soon" and still shows the missing definition. Similarly if you restore the definition, the outline gets updated too soon again and doesn't show the symbol.

So it seems like this fix will mostly apply to document symbols. But it should still be applied to all background requests to be safe, since different editors might behave differently.