Cancel (or not respond to) autocomplete if subsequent text is changed.

[abargnesi]

Changes to the BEL Statement input value should cancel (or not respond to) any in-flight completion promises.

The following describes the desired behavior:

1. Type some text.
2. debounce time passes; fetch completions in a promise
3. Type some more text.
4. The text change event should cancel the promise in (2) if it has not completed yet.

We may be able to tombstone the promise such that when it receives the completion response we can disregard it.

There may also be a work-around to cancel promises - see this gist.