'get_completions' handler is now 'completions'
We now handle the starting FileReadyToParse event

The problem was that when you start vim like "vim foo.cc", the FileReadyToParse
event is sent to the server before it's actually started up. Basically, a race
condition.

We _really_ don't want to miss that event. For C++ files, it tells the server to
start compiling the file.

So now PostDataToHandlerAsync in BaseRequest will retry the request 3 times
(with exponential backoff) before failing, thus giving the server time to boot.
Adding 'qf' (quickfix) filetype to ignore list
Added 'gitcommit' to filetype-completion blacklist
Diagnostics work again... somewhat.

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out-of-date diagnostics and then not updating the UI when things change.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.
Fixing the diagnostic-related race conditions

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).