kill tidb [session id] can't stop executors and release resources

[qw4990]

Feature Request

Is your feature request related to a problem? Please describe:

When users want to cancel a SQL, they may use kill tidb [session id].
But it can't stop executors since executors don't watch the ctx used to cancel the session and the cancelled session don't call executor.Close() to stop them.
So after kill tidb, these executors will continue to consume resources to do their job.

Describe the feature you'd like:

Background executors should be stopped quickly after kill tidb.

[qw4990]

I'm working on this now.

[laimisnd]

Meanwhile, is the systemctl restart tidb-4000.service a good enough solution to the query/session kill problem ?