MDL-35680 user selector: more usable when queries are slow.
The problem is to do when you pause a bit, so a search starts, and then
type a bit more before the search finishes. It was behaving like this:
1. Type a bit, then pause.
2. First search request sent.
3. Type a bit more, then pause.
4. Second search request sent.
5. First search request completes.
6. You try to select one result.
7. Second search request comples, overwriting what you are selecting.
This fix works by cancelling the first request before sending the
second.