Query cancellation fix

[k-anshul]

Hey @marcboeker

Found one tricky issue in the query cancellation that we had discussed previously.
It is easy to reproduce when query completes quickly like in case of in-memory DB and checkpoint query. Example below:

func main() {
	db, err := sql.Open("duckdb", "")
	if err != nil {
		log.Fatal(err)
	}
	defer db.Close()

	conn, err := db.Conn(context.Background())
	if err != nil {
		log.Fatal(err)
	}

	var wg sync.WaitGroup
	wg.Add(100)
	for i := 0; i < 100; i++ {
		v := i
		go func() {
			ctx, cancel := context.WithCancel(context.Background())
			defer cancel()
			defer wg.Done()
			_, err = conn.ExecContext(ctx, "CHECKPOINT;")
			if err != nil {
				log.Printf("err %v", err)
			}
			fmt.Printf("iteration %v done\n", v)
		}()
	}
	wg.Wait()
}

cc : @begelundmuller

[k-anshul]

Hey @marcboeker

Just nudging in case this missed your radar.

[marcboeker]

@k-anshul Thanks for the ping. Missed the change from draft to ready to merge.