After we started reporting progress when running cargo check during
loading, it is possible to crash the client with two identical progress
tokens.
This points to a deeper issue: we might be running several cargo checks
concurrently, which doesn't make sense.
This commit linearizes all workspace fetches, making sure no updates are
lost.
As an additional touch, it also normalizes progress & result reporting,
to make sure they stand in sync.
2ed258ba42