Provide a way to enable "rollback XA on disconnect" behavior we had before 10.5.2

Please, provide a way to enable the "rollback XA transaction behavior at disconnect" that we had before the fix for MDEV-742 (in versions < 10.5.2), at least at session level with a variable like:

set session rollback_xa_at_disconnect=true;

This may be useful for those who upgrade from 10.4 or older versions to 10.5+ and have application that expects the previous behaviour. Such applications end up with XA transactions hanging in prepared state now, when sessions got disconnected for whatever reason, and eventually block something (and this is when they are usually noticed).