This implements an asynchronous message system using a long-poll request
to the server.
The server provides an endpoint /receive_messages which blocks until
either a timeout occurs or we receive a batch of asynchronous messages.
We send this request asynchronously and poll it 4 times a second to see
if we have received any messages.
The messages may either be simply for display (such as startup progress)
or diagnostics, which override the diagnostics returned by
OnFileReqdyToParse.
In the former case, we simply display the message, accepting that this
might be overwritten by any other message (indeed, requiring this), and
for the latter we fan out diagnostics to any open buffer for the file in
question.
Unfortunately, Vim has bugs related to timers when there is something
displayed (such as a "confirm" prompt or other), so we suspend
background timers when doing subcommands to avoid vim bugs. NOTE: This
requires a new version of Vim (detected by the presence of the
particular functions used).
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