Allow force-quit if a transaction is running

[maximsmol]

Description

Currently if pgcli detects an active transaction, it will not allow quitting with ^D or the quit command. This means the user is stuck until the transaction finishes. I've been working around this issue with bash job control (^Z to suspend, then killing the process manually).

Note that the ROLLBACK option does not work since the connection has a query in progress:

> select pg_sleep(60)
^C^Ccancelled query
>^D
A transaction is ongoing. Choose `c` to COMMIT, `r` to ROLLBACK, `a` to abort exit. [a]: r
sending query failed: another command is already in progress
Time: 0.010s

This is particularly problematic in two cases:

1. Hanging queries that freeze the session for a long time (e.g. pg_sleep(3600))
2. When a connection has failed but pgcli does not realize it has to reconnect (e.g. after the system goes to sleep for a long time while pgcli is running, like on a laptop)

This patch simply adds a force option to the in-transaction exit dialog that allows exiting annyway.

Checklist

• I've added this contribution to the changelog.rst.
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