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out-of-date diagnostics and then not updating the UI when things change.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).
This is still fast & efficient because if we detect that the buffer hasn't been
changed (by examining b:changedtick), the parse doesn't proceed.
In effect, we now make sure we parse the file after every change to the buffer
as soon as that change happens. This means that compilation error feedback will
now be much, MUCH quicker.
