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).
By toggling the g:ycm_seed_identifiers_with_syntax option, the user can now tell
YCM to seed the identifier database with the language's keywords.
This is off by default because it can be noisy. Since the identifier completer
collects identifiers from buffers as the user visits them, the keywords that the
user cares about will already be in the database, regardless of the state of the
new option. So the only keywords added will be the ones the user is not using.
Meh. But people want it so there.
Fixes#142.
