Instant Search: add cancellation logic for outdated fetches

From https://github.com/Automattic/jetpack/pull/12741#discussion_r299373087:

> Slow connections and unreliable connections are the reason you can't rely on debounce to protect against race conditions. If request 1 fires off, and then half a second later (a typical debounce timeout) request 2 fires off, there's no guarantee they'll resolve in that order. On fast networks they probably would, but on mobile and other networks that are quite variable in speed, there's no guarantee.

(This was originally part of #13027.)