Cancelling SQL cell execution blocks Mimir

[okennedy]

Communication between Vizier and Mimir is synchronous, meaning that there's no way to asynchronously abort execution of a running Mimir query. The associated Vizier task aborts properly, but mimir keeps processing the query, blocking access to data until it finishes.

[okennedy]

A simple way to address this would be to add some sort of sentinel keeping an eye on the connection state. If the connection is killed, then we kill the worker.