Async (native) backend with cache disabled flickers when user types too fast

[dgutov]

This is basically a remaining issue from the previous work on asynchronous company backends (and improving the popup display when they are used).

Linking to the old comment: #654 (comment)

To reproduce:

• Add no-cache t to the definition of company-clang.
• Replace the clang executable with a script like (to make sure responses are slow)

#!/bin/env bash

sleep 1

exec -a $0 /usr/bin/clang "$@"

The current practice in this field is returning the previous list of completions when the computation of the new one is interrupted. Perhaps this can be good enough for the built-in fix as well.

Additionally, I've got a request for opt-in wrapping of capf sources with while-no-input. It seems like, without additional improvements, such sources should exhibit the same problem, so the fix could be the same.