Process notifications asynchronously

[crtschin]

Would close #4725.

Implements the ideas from #4725 (comment). Makes the following changes:

1. Processes notifications asynchronously keeping with the logic that notifications preceding any request, also finish before that request is processed. But does so asynchronously.
2. The above opens up the possibility that there could be multiple shake restarts in-flight because multiple notifications arriving in quick succession. Instead of storing these restarts in a queue, accumulate all restart changes in a single slot, to be processed at once.

[crtschin]

I can't get tests to pass with notifications fully async. Kinda makes sense looking at what these notifications indicate. I thought I could get away with only LSP handling everything synchronously of the VFS-side, but it appears not.

I'm now trying out a small variant where notifications are still synchronously, but that the session restarts are initiated async, while still blocking subsequent LSP requests. So restarts still get squashed.