Make computed.sort generate an answer immediately

This is a followup to pull #9356 that gives tighter timing. It extends the arrayComputed protocol with a `flushedChanges` callback so that implementations like `computed.sort` can confidently do batched
operations without waiting a whole run loop.

This is worth taking along with #9356, because otherwise we may breaks people's apps if they were expecting synchronous resolution.

cc @stefanpenner