fix: use the simple query protocol to execute BEGIN/COMMIT out of pipeline

fix: use the simple query protocol to execute BEGIN/COMMIT out of pipeline

We started using the extended protocol in e5079184 to fix #350, but,
probably to keep symmetry, we also changed the behaviour out of the
pipeline.

This turns out to be a problem for people connecting to the PgBouncer
admin console. They can use the `ClientCursor`, which tries to use the
simple protocol as much as it can, but they currently have to use
autocommit. With this changeset autocommit shouldn't be needed anymore.
See #808.