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.
My concern about `deferred:try` is the note in the deferred README about "The 'finally' task may not be called because of asynchrony." If I'm reading that part correctly, then what I've got should technically be more robust than using `deferred:try`. 

In any event, I've asked the author of `deferred` for some guidance:

    https://github.com/kiwanami/emacs-deferred/issues/29