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).
We could just remove the "dup: 1" part in the completion dict, but that would
leave the duplicate removal up to Vim which would be slow. Also, we might not
end up returning the correct number of results then.
