Single notify

We no longer allow concurrent notify/parse events to be sent to `ycmd` for a given buffer. We do this by setting a buffer-local flag indicating when one is in-flight, and unsetting that flag when the request ends.

Note that multiple notifications can still occur, but they will all be for different buffers. I think the congestion this could cause is more a problem in principle than in practice.
s/is/it. Also, any reason to not use deferred:try, which sounds like it would guarantee what you want
I'm not really fluent at elisp yet, but it seems to me that given that we have nextc and error, the only thing not caught is cancellation?