Fix #277: Send exit as a notification

[nemethf]

This is what the specification requires. @PerMildner, thanks for
reporting and analyzing the issue.

• eglot.el (eglot-shutdown): Use notify instead of request for the
  exit LSP method.

[joaotavora]

Hmm. Ok. But let's just be aware that this is changing the timings for a shutdown. It shouldn't make any difference, but the line below will probably not "return immediately" anymore. It is now jsonrpc-notify that returns immediately (but there's no need to write that fact).

So maybe just delete the comment on parenthesis. Also let's keep @ilohmar in the loop if he isn't yet.

[ilohmar]

Thanks for mentioning me. I cannot see anything wrong with the change, but as I said elsewhere, I have never dug deeply into jsonrpc at all.

[joaotavora]

I have never dug deeply into jsonrpc at all.

I recently double-checked that jsonrpc-shutdown doesn't exit with a live process. (I think at least, I put a lot of effort into making it so). So I think we should be fine. @nemethf do the change and push at will.