Cancelling query send does not cancel query

I'd expect ctrl-c or ctrl-d that cancels a query in the foreground, to also cancel that query on the backend (the postgres server).

for example:

```sql
postgres> SELECT some_long_running_query();
^C^Ccancelled query
postgres> select 1;
sending query failed: another command is already in progress
Time: 0.004s
postgres>
A transaction is ongoing. Choose `c` to COMMIT, `r` to ROLLBACK, `a` to abort exit, `force` to exit anyway. [a]: a
postgres> select 1;
A transaction is ongoing. Choose `c` to COMMIT, `r` to ROLLBACK, `a` to abort exit, `force` to exit anyway. [a]: r
sending query failed: another command is already in progress
another command is already in progress
another command is already in progress
another command is already in progress
Time: 0.004s
```

Reproduce the Ctrl-C or Ctrl-D sequence from the issue against a PostgreSQL server, then trace pgcli's foreground query cancellation and subsequent command handling. Done means cancelling the foreground query also cancels it on the backend, so a later `select 1;` does not fail with another command already in progress.

Scritto dal modello di indicizzazione a partire dal testo della issue.