Now, every FileReadyToParse event returns diagnostics, if any. This is instead
of the previous system where the diagnostics were being fetched in a different
request (this caused race conditions).
There appear to be timing issues for the diag requests. Somehow, we're sending
out-of-date diagnostics and then not updating the UI when things change.
That needs to be fixed.
The server is multi-threaded and will spawn a new thread for each new request.
Thus, the completers need not manage their own threads or even provide async
APIs; we _want_ them to block because now were implementing the request-response
networking API.
The client gets the async API through the network (i.e., it can do something
else while the request is pending).
