Cancel a running query.

[mingmwang]

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In current Ballista implementation, when a SQL is submitted, there is no way to cancel it. We need to implement a way to cancel a running SQL, and if there are running tasks scheduled to run on Executors, those running tasks should be cancelled as well.
I'm not sure whether DataFusion can cancel a running query or not. For Ballista, it needs additional communication to cancel running queries.

1. Add cancel_query rpc call to Scheduler grpc protocol
   rpc CancelQuery (CancelQueryParams) returns (CancelQueryResult) {}

2. Add cancel job logic to query stage scheduler

3. Cancel the running task on Executor

4. Cancel the running Futures on Executor's Tokio queue

5. Check all the Datafusion async spawn logic, make sure the spawned future can be cancelled.

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