`SHOW task_group` displays `RUNNING` status after coordinator crash

**Problem**: after executing the `STOP TASK_GROUP ` command, the `SHOW task_group` command continues to display the status as `RUNNING` for that task group, instead of showing it as stopped.

**Why**: the `STOP TASK GROUP` command only sets a stop flag in memory (`StopMoveTaskGroup[id] = true` in [qdb/memqdb.go:1228](https://github.com/pg-sharding/spqr/blob/master/qdb/memqdb.go#L1228)), but does not update the status in `TaskGroupIDToStatus`.

The status is read from `TaskGroupIDToStatus` when executing `SHOW task_group` (see [pkg/engine/virtual_relation.go:575-591](https://github.com/pg-sharding/spqr/blob/master/pkg/engine/virtual_relation.go#L575-L591)), so it remains `RUNNING` until the process actually processes the stop flag.

**What to do**: The `STOP TASK_GROUP` command should update the task group status to `STOPPED` (or similar) in `TaskGroupIDToStatus` so that `SHOW TASK_GROUP` displays the correct status of the stopped task group.