The recent completion rewrite makes completion faster for a large number of cases, but it has a couple of issues:

 * Any completion provider that needs to make edits beyond the few we've special cased doesn't work correctly. Roslyn is looking to add more of these, such as dotnet/roslyn#47511.
* Completion providers that we do special case can be _slow_.  Override and partial method completion in big types is painful, taking over a minute to show up sometimes.

To resolve this, I've added support for async edits by adding a new completion end point: /completion/afterInsert. It takes the item that was inserted, the file, and the new cursor position. It relies on the InsertText that was inserted still allowing for resolving the completion in the same way as before, so I had to force a few providers that don't behave well here to be eagerly resolved, regardless. I also kept import completion using the standard /completion/resolve step to resolve extra edits, as they don't need to modify the current cursor location at all.