atexit won't run registered functions for SIGTERM which we send to the server.
This prevents clean shutdown.
Also making sure that the server logfiles are deleted as well.
This means we can now load just ycm_client_support (which is a much smaller
library) into Vim and ycm_core into ycmd. Since ycm_client_support never depends
on libclang.so, we never have to load that into Vim which makes things much,
much easier.
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out-of-date diagnostics and then not updating the UI when things change.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).
