Real query cancelling support in spqr-console

[reshke]

do real cancelling

https://github.com/pg-sharding/spqr/pull/1388/files

[Denchick]

Current behavior: After #1388, spqr-console sends a cancellation key, but it does not actually stop the running query.

Expected behavior: spqr-console should perform a real query cancellation, terminating the query on the backend and freeing resources.

Why important: Avoids wasted resources and ensures that “cancel” actually stops execution.