fix(lsp): only call notify_reply_callback once the request is actually resolved

[saecki]

This is a follow up to #167

The complete function might do some async work, such as fetching crates.io versions or searches.
If I understand correctly a lsp request has 3 states pending, cancel, and complete.
The documentation says to call the callback once the request is no longer pending, which (because we don't handle cancellation) would make it complete right?

So this just schedules the notify_reply_callback after the callback has been called by the request handler.

Are my thoughts correct here, or is the no longer pending more of a: when work has started?
@PolyMeilex I would appreciate your input on this manner :)

[saecki]

Oh, never mind the notify_reply_callback is for the synchronous case...

[PolyMeilex]

Yep, since neovim/neovim#33624 it is synchronous aware.

[saecki]

So just to clarify, should notify_reply_callback be called once the server has begun handling the request, or after the request has been completed, thus the callback has been called?