Performance - select2 based contact autocomplete spams the DB server with potentially long running queries

Noted in a 70,000+ contact civicrm database while trying to add a shared address to an organisation.

The autocomplete field for shared address does not wait for any results or failure indication before sending the next request, which in our testing push the mysqld instance to 250% CPU usage in 30 concurrent queries.

The user experience in this instance was that the software appeared to have frozen on the throbber indefinitely.

Ideally the search function could first kill any running queries from the same (PHP / CiviCRM) session - although if it's the case the DB_Dataobject really doesn't have any capability for async queries, it may be more feasible for the front-end AJAX to wait for an existing query to return before requesting the next one.