Mechanism to kill currently executing queries in Query service

There should be the ability to stop a currently executing query that is present in active_requests from the Query service (server-side).

If long-running queries are in danger of affecting Query service stability, such as causing memory usage to grow to a point where we are in danger of resource exhaustion, a user should have the ability to cancel query execution on the server-side to recover from this situation instead of having to kill the Query service itself.